Introducing GravityBox Actions
Interface for providing GravityBox specific actions to third party apps.
This will also allow us to use special actions within App Launcher /
Navigation ring (e.g. such as show notifications/quick settings panel,
switching network modes, etc...)

This initial release comes with one action implemented: Show Power Menu.
Additional actions can be added rather simply by defining new shortcut
class extended from abstract shortcut class and implementing behaviour.