ENH: Add second pimpl constructor to qMRMLTreeView

Adding a second protected constructor that takes the pimpl instance as argument allows subclassing and using qMRMLTreeViewPrivate from a subclass of the tree view.

Export qMRMLWidgets_INCLUDE_DIRS CMake variable from SlicerConfig to be able to include those directories without CMake path construction magic.

From: Csaba Pinter <csaba.pinter@queensu.ca>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22883 3bd1e089-480b-0410-8dfb-8563597acbee