Change xsge_physics.Wall to xsge_physics.Solid

Closed username-removed-876611 requested to merge (removed):master into master

Change all instances of xsge_physics.Wall to xsge_physics.Solid, so that the current pip version of xsge_physics doesn't cause the game to crash as soon as the fire key is pressed.

xsge_physics.Wall doesn't exist, and the most logical element to replace it, in xsge documentation, seemed to be xsge_physics.Solid. It works, so I assume it is what was intended. Given that the hexoshi developer is also the xsge developer, any additional insight would be appreciated.