Add mouse support

[tobiasvl]

Adding mouse support à la Polarium's touch support would be nice for mobile users.

[tobiasvl]

It's coming along. Just some notes for my own sake here; minor stuff I still need to fix:

• Collect all mouse-related stuff in the mouse table (Clean up the codebase #16)
• Proper touch screen support
• Mouse support in the level editor
• Mouse support in the password editor
• Don't register mouse click right after selecting a level in the level select
• Get rid of mouse displacement when selecting some levels (camera manipulation really getting back to bite me in the ass here)
• The marker will appear at the default position for one frame when selecting a level, before it jumps to the mouse position
• Clearing/failing a level and then choosing "Back" will select the level under the cursor (not resetting the mouse click status correctly before returning to level select)
• Go back/forward in states (especially between the different editor screens) with the mouse/touch