Fixed button system, model changes

Interior door sticker/sign now matches exterior. Fixed and implemented
'double trace' from legacy (properly this time). Going into a tardis
while inside a tardis now works better. Added a basic clientside sprite
test (thank you vcmod for basic method). Players can now exit using
Alt-E on the door instead of interior. Parts system now generates actual
entities using base classes instead of using hacky table merging
techniques. Fixed bug with multiple TARDISes breaking parts. Updated
interior door frame uv wrap and texture.