mathutils: use hypot() for distance calculation.
ui: get rid of home-(under)grown GUI.

In preparation for switching to SFGUI. As interesting as implementig a
GUI is, that's not what this project is about.
Add SFGUI to build system.
render: split clearing into a separate function.
GameState: move tick_clock_ to the base class.

Will be needed by more states soon.
Add some more logging.
App: call OnExit() on final state before quitting.
Implement main menu using SFGUI.