Added - Tile Edit Mode

Opening the tile editor now enables edit mode, which allows the focus to
move away from the player to alter other tiles.
- Can change any tile by entering edit mode (TAB key) and clicking the
desired tile's image
- Extra tile images set the tile to empty, even though they have an
image in the UI
- If player tile is overwritten, it'll be recreated when exiting edit
mode (as whatever tile is selected last)
- F5 key saves the map
- move up/down in a layer by pressing 'r' and 'f' keys respectively