Clean up PingusExit

[Wuzzy2]

PingusExit is a bit messy right now, and it can not be used as-is, neither in the scene nor in the toolbox.

PingusExit needs some cleanup so it becomes useful by default.

The following things need to be done:

• [x] Default name for user interface (I suggest “Exit”—short and simple)
• [x] Default tooltip (I suggest “Penguins will escape here, never to be seen again.” ;-) )
• [x] Sane default size (Size used in Pingus level is a good reference)
• [x] Default image (opendoor should do. But the object would still be open for custom images, I guess)
• [ ] Clean up existing levels using PingusExit
• [x] Use appropriate default Z-level so it is in background by default
• [ ] Move sensor to bottom so the exit works even if it is very large

[kaa-ching]

Agreed, it was a bit of a hack. Let's fix it before there are too many of them.