Fixed scene tools not getting notified when all items are being deleted.
Fixed From/To command crashing when a map didn't have required tilesets.
Fixed CellDocument crash when map's were reloaded but the scene hadn't been set.

During MainWindow::documentAdded(), the document has been created before the
scene has been assigned, then the CellScene calls loadMap(), which puts up
a progress dialog, which causes a mapAboutToChange() signal from somewhere.
So don't connect the signals until the scene's been set.
Fixed MapManager::loadMap() hang when recreating a MapComposite.

The sub-maps weren't being loaded asynchronously.
Fixed CellScene not synching layer groups when a sub-map finished loading.

Now that MapComposite loads sub-maps asynchronously, and addMap() sets the
mNeedsSynch flag, there needed to be a way to tell CellScene to update itself.
Allow Blend.blendTile to use an alias name.
Fixed MapManager::loadMap() hang.
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.