Lot of (quite) invisible work

In the last two weeks some non-visible work was done on Jatris.

- Added preliminary code to support non squared grids
- Added code to support holes (non swappable positions) in grids
- Done some code cleanup
- Done some graphics for a (possible) HD/Tablet version (1280x800)

Also finally completed bgm code (now it seems to work right).

Maybe all this work may lead to a new version (or a brand new game)