Added support to make an entity translucent, and used it to show when a boulder trap is deactivated.

Fixes #294
Applied the code guidelines on the GameEntity class.

Part of #91
Compilation fix after rebasing.
Apply fixes/improvements suggested by @hwoarangmy :)
Adapted the code to handle activation per trap tile.

As discussed with @hwoarangmy. :)
Merge pull request #342 from Bertram25/boulder

Added support to make an entity translucent (For the boulder trap) + game entity class cleanup.