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, and instead forward CTK_INSTALL_LIB_DIR to
QtTesting_INSTALL_LIB_DIR.

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