Folders: Merge Folders

This allows a folder A's contents to be merged into folder B
when folder A is drag/dropped onto folder B.

Change-Id: I41d62d04b6b4f0932cc68fb6d2a3be111b063d53
Trebuchet : Add folder on folder checks

- Check if the new folder can accomodate all items
- Don't expand folder if trying to drop folder on folder

Change-Id: I208e3afd4ee6aeeea01928fc77e136d6085c4fdc