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