Refactor edit_cache tool selection to ask each global generator for its
preference.  Teach the Ninja generator to always use cmake-gui because
Ninja by design cannot run interactive terminal dialogs like ccmake.
Teach the Makefile generator to use cmake-gui when also using an "extra"
generator whose IDE has no terminal to run ccmake, and otherwise fall
back to CMAKE_EDIT_COMMAND selection for normal Makefile build systems.