ENH: Implement downloading of extension icons

Change qSlicerExtensionsManageWidget to attempt to download icons for
extensions that don't already have an icon on disk. On successful
download, the icon is saved and applied to the extension list item.

Note that we will retry downloads for missing icons on first showing of
the extension manager. This seems like reasonable behavior. Missing
icons fall back to the default icon as before, and download errors are
silent.

Issue 0001907

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23202 3bd1e089-480b-0410-8dfb-8563597acbee