Refactor MIME DB access

* Add class DatabaseLocation. It contains a list of the MIME DB
directory paths plus methods to access type files.
* Move all low-level MIME DB access functions from
database_{support,access} to DatabaseLocation. All code that formerly
used those now requires a DatabaseLocation object. In BMimeType and in
the registrar the default object is used, but the low-level classes
can now be reused with different locations.
* Move get_icon_data() from database_access to database_support and
delete the former, which is now empty.

Move MIME DB code back from registrar to src/kits/storage/mime

* Together with database_{access,support}.cpp it is built into a static
library.
* Add new interfaces MimeSniffer and Database::NotificationListener for
plugging in registrar specific functionality (the sniffer add-on
support and the notification mechanism).

Refactor MIME DB access

* Add class DatabaseLocation. It contains a list of the MIME DB
directory paths plus methods to access type files.
* Move all low-level MIME DB access functions from
database_{support,access} to DatabaseLocation. All code that formerly
used those now requires a DatabaseLocation object. In BMimeType and in
the registrar the default object is used, but the low-level classes
can now be reused with different locations.
* Move get_icon_data() from database_access to database_support and
delete the former, which is now empty.

Move MIME DB code back from registrar to src/kits/storage/mime

* Together with database_{access,support}.cpp it is built into a static
library.
* Add new interfaces MimeSniffer and Database::NotificationListener for
plugging in registrar specific functionality (the sniffer add-on
support and the notification mechanism).