Bugfix for editor and play, usability improvement for editor.

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. Previously, clicking a non-selectable element (like Grass) on the left thang list would select the element, enabling it to be dragged and repositioned. Fixed this.

3. 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.