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.

**CAUTION:**
The QtTesting changes break Slicer, and thus require that mwoehlke-kitware/Slicer@76721f171183962220b7d75f4716b6fa9b4715a5 be merged at approximately the same time. Please coordinate with me so that this can be accomplished.