Add 'rules' to install from superbuild

Add a script called when installing from the superbuild build tree that
installs the various external projects, and add necessary rules to
register the same with said script. Remove build-time install of
QtTesting (which does not need to be installed to be used) so that it
may be used with this system.

Note that this support (currently) excludes DCMTK, KWStyle and PythonQt.