Bugfix for play, usability improvement for editor.

Resubmission of #561. I'm repeating the comments from there - 

1. On the thang tab in the level editor, panning to the thang clicked in the thang list on the left makes the editor easier to use. It works fine, but I'm not sure about the way I've implemented it, code-quality wise

2. Moving the wizard around with arrow keys allows the player to move the wizard out of the level, which is quite counter-intuitive. Added checks for that.