Level fixes & clean-up

* This fixes the last of the issues for the Greendale levels from #1488
* Fixes some of the issues in #1497 (Village forest & treeline)
* Adds isometric arrows to doors in the side walls, fixing #1601
* Reorganize most of the images which were just dumped in the images folder - now in sub-folders either by level, platform or liquid. I've left the valley levels alone.