Also, fix up inclusions, mark properties for translation, etc.
Makes explicit the fact that you are interacting with the individual action rather than the group and removes potential namespace conflicts with classes implementing the interface (like g_application_activate() vs g_application_activate_action()).
- make GAction.get_state() return a reference - fix some leaks/warnings in the tests - fix signal propagation in GSimpleActionGroup
and a simple test
