Adjust Database{Location} to only attempt to create a mimetype when
actually necessary, and fail otherwise if a writable version doesn't yet
exist. Correspondingly, adjust callers such as
DatabaseLocation::DeleteAttribute(). Fixes a problem where a caller asking
to perform a mimeset could fail early due to SetSupportedTypes() attempting
to update the read-only mime database entry supplied by a package, and
consequently most of the mimeset operations would be skipped.
* 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.
