add more user feedback for misplaced objects in the Level Editor

[evemartin]

Summary
At the moment, when objects in the Level Editor like traffic lights and cows are dragged around the map, users get feedback about where they can be placed via highlighting: valid squares are highlighted in green and invalid squares are highlighted in red. This task is to determine whether any more user feedback should be provided when an object is misplaced (i.e. placed on an invalid square) and, if so, to implement those changes.

User story
_As a Rapid Router player
_ I would like to know if I have placed my object on an invalid square
_so that I can adjust my level to make it valid