Action bar added

Underground Stuff now has an action bar. Left click performs the selected action, rightclick deselects the action.
1 and 2 to select the corresponding action. The third slot is unassigned, for testing purposes.

Currently the only actions are 'dig' and 'place wall', more will be added in the future. (some stairs or a ladder would be handy)

It will ofcourse be possible later to freely assign actions to your action bar and set your own hotkeys if you so desire.
Before I spend time on making menu's to do that, there has to be a game loop first.

changelog:

• action bar added
• terrain is now only highlighted when relevant
• mouse icon changes with the currently selected action