Improvements in moving around in the level editor

Minor stuff that came up in the [usability tests](https://docs.google.com/document/d/14Q_LADxB2LGuPLCZgCXijWrugAsdKtwq0L9SBNyXeMY/edit) - 

1. Disable right-click panning
2. Allow moving around the map when an addThang is selected