Cleaned up some strange MapObjectModel code

The MapDocument pointer of the model should not be changed. The model
is owned by the MapDocument after all. Also, there was no need for
the additional (and uninitialized) mMapObjectModel member.

Coverity-issue: 1165682 (Uninitialized pointer field)
Added support for moving image layers in the Properties window
Flare plugin: Removed unneeded null pointer check

Coverity-issue: 1165680 (Dereference before null check)