Icon-O-Matic: Document the document saver classes

Change-Id: I1985108bbc4e1030b2dc0e94cfb1c0706b1806a4
Reviewed-on: https://review.haiku-os.org/c/haiku/+/6442
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>
Reviewed-by: Adrien Destugues <pulkomandy@pulkomandy.tk>

big clean up of Save and Export stuff:
* implemented SavePanel that let's you chose the Export format
* introduced DocumentSaver class and several implementations
* Document no longer remembers any entry_refs, but a native
and export document saver, for other export formats than
HVIF, it is no longer necessary to go through the file panel,
simply invoking Export will do
* this makes it much easier to have a custom saver that sends
a message with an HVIF icon to another app instead of saving
to a file
* cleaned up File menu
* last used export format is persistent


git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@19307 a95241bf-73f2-0310-859d-f6bbb57e9c96

big clean up of Save and Export stuff:
* implemented SavePanel that let's you chose the Export format
* introduced DocumentSaver class and several implementations
* Document no longer remembers any entry_refs, but a native
and export document saver, for other export formats than
HVIF, it is no longer necessary to go through the file panel,
simply invoking Export will do
* this makes it much easier to have a custom saver that sends
a message with an HVIF icon to another app instead of saving
to a file
* cleaned up File menu
* last used export format is persistent


git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@19307 a95241bf-73f2-0310-859d-f6bbb57e9c96