Refactor key mapping

This is the first small step of my global UI refactoring. In future I'm thinking of some additional main scene modes (look mode, ability target mode, maybe more) and UI actions that require additional information from player (such as view or change inventory in new "modal" inventory scene).

Key map will be in future transformed to `Key -> UIAction`; our old good `Action` will be implicitly convertible to `UIAction` for sake of simplicity.

And, of course, it will be user-configurable as stated in #32.

@Akon32, @rexim, someone, please review this.
Idea had been showing a warning here, but I decided to ignore it...