Small tweaks

Fix: handle HP and Morale <0 in drawing health bars
Change: remove/comment out some Debug code no longer needed
Change: wrap TileArray access in MapContoller.MortarHit in try/catch to
hunt down null reference exception
That took waaay too long.

Add: wall sprites and prefabs
Add: TileHelper class to contain common tile methods
Change: rename Tiles enum to Tile
Change: a few other tweaks
Refactor: make MapController tile methods be specific to trenches