Fixed issue with non-existing files

How to reprodue :
-Make boardList.txt containt a file name that refers to a file that
doesn't exist.
-Start a two player game
-Clear the Board
-The new Board will only load for the oponoent

Fix:
-Added remove calls to PhysicsManager to make sure all refences to the
assets are set to null, freeint the object thet sharet_ptr is pointing
to.
-Removed sending of LevelDone message in GameManager::GenerateBoard when
no Tiles where loaded.
-Fixed GameManager::IsTimeForNewBoard to check for local Player Board
empty OR ( two Player and opnoent is done wiith level )
-Added PhysicsManager::Clear() that clears all lists in PhysicsManager.