Determine behavior when tilesets are removed

[albin-johansson]

Currently, when a tileset is removed, the tile layers remain untouched. This means that they may contain invalid tile identifiers.

There are several ways to deal with this. We could add an action for fixing invalid tile identifiers, going through the tiles and setting invalid ones to be empty. Alternatively, the command responsible for removing tilesets could cache all tiles from the tileset, and subsequently set them to be empty. This approach would always keep the map in a "good" state, and allow for better state restoration upon undoing the tileset removal.

Since it's possible to import maps with invalid tiles, we should probably pursue both strategies.

Requirements

• Add "Fix Invalid Tiles" action
• Make tileset removal command cache the affected tiles, and restore them as needed