Add in-game menu for level selection + options

We currently rely on the old PyGTK jumpnbump_menu for that, but it would be much cooler to have a proper in-game menu. It should give the possibility to change the level, and ideally also to tweak some options currently only available on the command line.

As the current player selection "menu" makes clever use of the gameplay (you move the players you want to play to the right side of the menu "level"), it would be nice to have the level selection and maybe option configuration implemented in a similar manner.

A proposal (could make a mockup if needed):

• The current player selection level could have a pixel art frame (maybe made out of branch/vine?) added that would display a preview of the currently selected level. Some kind of button could be activated by jumping on it to toggle the level selection.
• For the options, there could be an additional level screen to the left with several jumpable buttons toggling the exposed options.

So in short the idea would be to make use of the platformer mechanics to configure some stuff while keeping the game immersion, in the spirit of the current player selection menu. It would of course be a bit unwieldy, but provided you don't need to change options everytime you play, it should be fine. We also avoid having to create or embed a GUI library :)