ENH: Always return to extensions list

Tweak extension manager to navigate the manage browser back to the
initial page on changing tabs, so that when the user changes to the
'install' tab and back to the 'manage' tab, they always see the list of
installed extensions.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23173 3bd1e089-480b-0410-8dfb-8563597acbee