ENH: save the last sceneview thumbnail type

Keep track of the last scene view and screen snapshot thumbnail type
and use it for the next one that's being captured.
For the scene views, base the next scene view name on the last
one instead of always going back to the default SceneView.

Issue #1760


git-svn-id: http://svn.slicer.org/Slicer4/trunk@22929 3bd1e089-480b-0410-8dfb-8563597acbee