Image model and the media edit dialog

Creating an MWImageModel that handles all image edit operations.
If the new attributes mean a change of image types from inline
to block or vice versa, the model will handle the creation and
insertion of the new node.

Change-Id: Ibe71bc8bd74e4ba5a024ac722432ccf0b8f65e71
Merge "Image model and the media edit dialog"