ENH: Add access to the currently selected module for testing convenience

This commit adds a Q_PROPERTY to qSlicerModuleSelectorToolBar to ensure it is
available from python. It also adds the convenience method to the
slicer.util python module.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23799 3bd1e089-480b-0410-8dfb-8563597acbee