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 0003045

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22347 3bd1e089-480b-0410-8dfb-8563597acbee