Added quick incomplete readme, and GPL v3 license file
Fixed issue where sprite graphic selector in placing sprites dialog
allowed selection of off-by-one values.

Placing sprites dialog has all values auto filled in, so no empty
strings are for starting out. Still needs better validation.
Ammending last commit: forget to include readme and licence
General refactoring over old parts of the code base. Game can now
actually be played again (since focus was put on level editor). Playing
the game involves choosing a level; no menu system yet.
Weird bug during death though that just breaks everything.