cmake-gui: Need field to set CMAKE_GENERATOR_INSTANCE
VS 2017 and above support multiple installation instances on a single machine. CMake 3.9 and 3.10 allow instance selection only via an environment variable. Since !1394 (merged) we now use a
CMAKE_GENERATOR_INSTANCE to save the selection persistently, but setting it in the first place must currently be done either via the environment variable or explicitly with a cache entry.
When a user in cmake-gui selects a VS 2017 (or above) generator, we should offer a field to select which instance of VS to use. We can update
cmVSSetupAPIHelper to enumerate available instances to populate a drop-down selection widget.