Double-clicking a tile in the Tile Layers Panel selects it in TileDefDialog.
MapComposite::recreate() loads sub-maps asynchronously.

This method is almost identical to the constructor which already did this.
Fixed MapManager::loadMap() hang.
Allow Blend.blendTile to use an alias name.
MapComposite emits a signal when it adds a sub-map asynchronously.
Fixed sub-maps in adjacent maps not displaying all their levels.

addMap() doesn't call ensureMaxLevels() when creating or recreating the
MapComposite, so we have to manually set mMaxLevel.
Fixed QGraphicsScene warning about removing an item that wasn't in the scene.