Change all action callbacks so they can be invoked by a GAction:
- add GimpActionCallback typedef:
void (* cb) (GimpAction*, GVariant*, gpointer)
- change all action callbacks to the GimpActionCallback signature
- add "gimp-activate" and "gimp-change-state" signals to GimpAction,
with the same signature as the resp. GAction signals
- remove all other custom action signals and only use the new
GimpAction signals
- pass around appropriate GVariants containing booleans, int32,
strings
- badly hack around to force a GimpProcedure pointer into a
uint64 variant
- remove all G_CALLBACK() casts from all action callbacks,
they all have the same signature now
Step one: get rid of all those deprecation warnings that make
it hard to see any other warnings:
- add a lot of dummy API to GimpAction, GimpActionGroup, GimpUIManager
etc. which simply forwards to the deprecated GTK functions, they
will all go away again later
- rename GimpAction to GimpActionImpl
- add interface GimpAction that is implemented by all action classes,
creates a common interface and allows to remove some duplicated
logic from GimpToggleAction and GimpRadioAction, and at the same
time adds more features
