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
