GLOBAL:
 * options : different types of tiles ?
 * background image + more animations
 * key profiles (?)
 * autopause if focus is lost (cf kmines)
 * set the keyboard repeat rate (possible ?) or better do not use the
   repeat...
 * use KGame


KNOWN BUGS:
 * occasional crash in multiplayer wizard / with keybinding changes.
 * infinite loop with out-of-bound call to GArray::at
   sometimes when creating several boards (human vs AI/human).
 * does not update immediately second board when changing background.