cmake-gui: Reference LGPLv3 when redistributing Qt

Download http://www.gnu.org/licenses/lgpl.txt and place it as
Licenses/LGPLv3.txt in our source tree.  When building cmake-gui, use
option CMake_GUI_DISTRIBUTE_WITH_Qt_LGPL to enable notification in the
"About" dialog of how the distribution of Qt is licensed.  Install the
license file as ${CMAKE_ROOT}/Licenses/LGPLv3.txt so that the dialog can
display a path to it.