cmake-gui: Need field to set CMAKE_GENERATOR_PLATFORM
cmake -A <platform> option was added,
CMAKE_GENERATOR_PLATFORM is now the preferred way to select the target platform instead of adding it to the VS generator name. However, cmake-gui still lists all the generators with the per-platform name suffixes and does not offer a field to set the platform name.
See discussion in !1215 (merged).
We already have one for