Merge topic 'cmake-gui-generator-platform'

67bced8a cmake-gui: Improve label for default platform
48ec0bc1 cmake-gui: Add field for generator platform selection
8bba458e Add global generator factory method to get default platform name
818df52c Add global generator factory method to get list of known platforms
8144b00e Split global generator factory list with and without platforms
b70c0aed VS: Factor out helper function to compute host platform name

Acked-by: Kitware Robot <kwrobot@kitware.com>
Merge-request: !2832