Expose "currently hovered app in doc" as variable to enable awesome dock shortcuts

This would let people emulate the Gnome dock and build things like

  • shift click on app to open new window
  • ctrl click on app to close app
  • ...
  • scroll over app icon to cycle through app windows (with the first scroll activating the app)

This variable must already be available internally to enable the "activate currently hovered app in doc" action.