Extensions work for brushes, dynamics, MyPaint brushes, patterns,
gradients, palettes and tool presets.
More to come, but this is a first and working proof-of-concept.
specified by GimpDataLoaderEntry structs. Remove the same code from
GimpDataFactory and make it an abstract base class that only serves as
an interface for actual implementations. Also move around some stuff
in GimpDataFactory and remove virtual functions that were a bad idea
in the first place.
