ImportUI MpaView : use Camera Thumbnails Contoller instead to use Camera...

ImportUI MpaView : use Camera Thumbnails Contoller instead to use Camera Controller to get image thumbs from camera.
As Camera Thumbnails Controller use a cache mechanism to store pixmap, this will prevent to query camera device each time.
This will increase map view speed, especially with gphoto2 based cameras.
Note : cache is populated initially with icon-view.
BUGS: 336333
FIXED-IN: 4.4.0