BUG: Fix missing icons in extension manager

Update qSlicerBaseQTGUI.qrc[1] to include icons for new tool menu in
extension manager. This fixes the button for the same having no icon on
systems that lack the themed icon, as the non-themed icon was being
specified and exists in the source tree, but was missing from the
aforementioned .qrc.

(1: http://www.slicer.org/slicerWiki/index.php/Documentation/Nightly/Developers/Build_system/Qt_resource_files)

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23479 3bd1e089-480b-0410-8dfb-8563597acbee