ENH: expand the view combo box to work on abstract view nodes

Removed the limitation on the display node view combo box that
only allowed selecting 3D view nodes, now all subclasses of
abstract view nodes can be used. Slice nodes and chart view
nodes are now allowed. This fixes the issue that while a
display node could have a slice node added to it's list of
view nodes, this combo box would remove it as it wasn't a
3d view node.
Update the volume rendering module widget to use the new combo
box code, limiting the view nodes to 3d ones since volume
rendering can't be shown in slice views.

Issue #3216



git-svn-id: http://svn.slicer.org/Slicer4/trunk@23086 3bd1e089-480b-0410-8dfb-8563597acbee