Mayhem - Implemented bombs

Bombs are now implemented. There is one on level 1 to play around with.
Press SPACE to drop it, it'll blow up any boxes or monsters in the area
surrounding it.

Other changes:
- Fixed the two issues from last night in the inventory canvas.
-Walls are now part of the same array as floors, they both bloated the
sprites array too much (on level 1 , they make up 203 out of 271 objects
for example).
-Minimap doesn't show the walls any more.
- Stars and Bombs are specific classes, not just spriteObjects.
- Added a removeObject function