BUG: Fix opening of link in extension manager

Opening of links different from slicer.kitware.com host into regular
browser has been initially introduced by r21897. Then, when commit r22220
introduced qSlicerWebWidget, the management of external links has been
broken.

This commit fixes the issue by ensuring the qSlicerWebWidget always open
link within its associated webview. The slot "onLinkClicked" from
"qSlicerExtensionsInstallWidget" will check what is the host and open
into a regular browser if needed.

See #3045

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22347 3bd1e089-480b-0410-8dfb-8563597acbee
BUG: Add back/forward button to extension manager

Fixes #2191

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22348 3bd1e089-480b-0410-8dfb-8563597acbee
STYLE: Tweak comment in SlicerConfig.cmake

See #3247

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22349 3bd1e089-480b-0410-8dfb-8563597acbee