IMAP: downloading mails is now working.

* Changed the way the attributes are written to make sure that everything
that can be written once is in fact written just once.
* The rename code in BMailProtocol::_ProcessFetchedHeader() was broken,
and caused the hang of the last commit.

Mail Kit: Changed how filter changes work.

* BMailFilter::HeaderFetched() now only alters the entry_ref, and returns
B_MOVE_MAIL_ACTION to move a mail.
* Instead of potentially moving the file around several times, the
BMailProtocol now takes care of carrying out the filter action just once,
including trying to make the file name unique.
* This also allows the IMAP add-on to know the final location of the mail,
and thus downloading a message actually works.
* However, with my test inbox, it currently hangs, and a current Debugger does
not work on my older system -- I guess I need to update.
* Replaced the duplicated space mechanism within the "HaikuMailFormatFilter"
that is substantially faster, and handles all whitespace, not just spaces.
It will also replace tabs with spaces.

Major mail protocol/filter settings rework.

* Instead of abusing BArchive::Archive() we now use a BMailSettingsView as
a base view for all filter/protocol settings that works with
BMailAddOnSettings.
* Cleanups in E-mail which is now completely layout friendly. But also still
crashes when changing the views.

Work in progress of mail rework.

* Not everything compiles; all protocols, and inbound filters do, though.
* Renamed a few classes to give a better idea what they are for; prefixed
public classes with the 'B' prefix.
* Moved ProtocolConfigView's classes into the BPrivate namespace.
* Moved BMailFilter into its own file.
* Added BMailFilter::DescriptiveName(). This is now used by the RuleFilter
in order to give a description of what it's doing (ie. no more dozens of
"Rule filter" entries in the preferences).
* Removed no longer used MailAddon.h.
* Renamed Addon to AddOn where found, since that is more consistent with the
rest of the API.
* Merged the former MailProtocol with the former MailProtocolThread; the
differentiation between those two was pretty messy.
* All configuration views touched so far are now using the layout kit.
* The RuleFilter is currently broken functionality wise; I have not yet decided
how to solve the stuff it uses (TriggerFileMove() does not exist anymore,
for example).
* BMailAddOnSettings (formerly known as AddonSettings) now directly subclass
BMessage; there are no Settings() and EditSettings() method anymore. The
class uses a copy of itself to determine whether or not it has been changed.
* Lots of cleanup.

IMAP: downloading mails is now working.

* Changed the way the attributes are written to make sure that everything
that can be written once is in fact written just once.
* The rename code in BMailProtocol::_ProcessFetchedHeader() was broken,
and caused the hang of the last commit.

Mail Kit: Changed how filter changes work.

* BMailFilter::HeaderFetched() now only alters the entry_ref, and returns
B_MOVE_MAIL_ACTION to move a mail.
* Instead of potentially moving the file around several times, the
BMailProtocol now takes care of carrying out the filter action just once,
including trying to make the file name unique.
* This also allows the IMAP add-on to know the final location of the mail,
and thus downloading a message actually works.
* However, with my test inbox, it currently hangs, and a current Debugger does
not work on my older system -- I guess I need to update.
* Replaced the duplicated space mechanism within the "HaikuMailFormatFilter"
that is substantially faster, and handles all whitespace, not just spaces.
It will also replace tabs with spaces.

Major mail protocol/filter settings rework.

* Instead of abusing BArchive::Archive() we now use a BMailSettingsView as
a base view for all filter/protocol settings that works with
BMailAddOnSettings.
* Cleanups in E-mail which is now completely layout friendly. But also still
crashes when changing the views.

Work in progress of mail rework.

* Not everything compiles; all protocols, and inbound filters do, though.
* Renamed a few classes to give a better idea what they are for; prefixed
public classes with the 'B' prefix.
* Moved ProtocolConfigView's classes into the BPrivate namespace.
* Moved BMailFilter into its own file.
* Added BMailFilter::DescriptiveName(). This is now used by the RuleFilter
in order to give a description of what it's doing (ie. no more dozens of
"Rule filter" entries in the preferences).
* Removed no longer used MailAddon.h.
* Renamed Addon to AddOn where found, since that is more consistent with the
rest of the API.
* Merged the former MailProtocol with the former MailProtocolThread; the
differentiation between those two was pretty messy.
* All configuration views touched so far are now using the layout kit.
* The RuleFilter is currently broken functionality wise; I have not yet decided
how to solve the stuff it uses (TriggerFileMove() does not exist anymore,
for example).
* BMailAddOnSettings (formerly known as AddonSettings) now directly subclass
BMessage; there are no Settings() and EditSettings() method anymore. The
class uses a copy of itself to determine whether or not it has been changed.
* Lots of cleanup.