Add 'rules' to install from superbuild

Import ExternalProject_Install_CMake from Artichoke (see 51fa9b43c1f2),
and use to install external projects when building the install target of
a CTK superbuild. Remove unneeded build-time install of QtTesting and
instead forward CTK_INSTALL_LIB_DIR to QtTesting_INSTALL_LIB_DIR.

Note that this (currently) does not install DCMTK, KWStyle or PythonQt.