Fix MWImageModel's getImageNodeType()

* Fixing a typo in the getImageNodeType() method ('frame' was supposed
  to be 'frameless').
* Adding a check for type 'mwBlockImage' before editing/adding captions
* Using the node's getType() method instead of the model's where possible.
* Adjusting default alignment, so that when we check if it is set, it gives
  us the actual answer, which is always useful.

Bug: 65568
Change-Id: I1687d8c7430c723b52448a9e24641e2c30d85d6a