Change GimpFileDialog's file-procs properties to enum
GimpFileProcedureGroup and get them via the newly added API. Set
properties common to all dialogs in the parent class.
file-procedure.[ch] is gone and its functions moved to
GimpPlugInManager where they belong (the manager keeps around the
lists of load, save and export procedures).
Utility functions from file-utils.[ch] that have nothing to do with
image files moved to core/gimp-utils.[ch].
Metadata presence implies XCF version 6 or higher. Yet metadata are
stored in parasites which are perfecly supported in older GIMP versions
and are passed along through saving.
Therefore there is no need to keep XCF 6. We just add a warning telling
that metadata won't be visible in older GIMP when saving a file with
metadata and compatibility mode.
By default, it only saves the current file filter, but it allows a child
to implement its own state. It has been implemented by GimpSaveDialog to
save the preferred compatibility mode as well.
The code refactoring allows better readability which will allow in turn
to add more complex specific features making good use of the save/export
split concept.
