BUG: Prevent extensions from being loaded twice when "Install" tab initially hidden

For details: https://github.com/midasplatform/slicerappstore/pull/36

See http://na-mic.org/Mantis/view.php?id=2911

Fixes #2911

git-svn-id: http://svn.slicer.org/Slicer4/trunk@21660 3bd1e089-480b-0410-8dfb-8563597acbee