Can't change Qt for iOS kit CMake generator from Xcode to any other generator

1) Start Qt Creator -tcs
2) Add an official Qt for iOS 6.6.1 qmake.
3) Make sure the run device type is either ios device or ios simulator
4) Edit the auto-generated kit, click CMake Generator Change -> Select Ninja -> Press Ok
5) Observe that the generator remains Xcode

Same happens with a custom created / non-auto-generated kit. The selection is just discarded.

The generator should be configurable, for the use case of building qt submodules. In that case require usage of Ninja / Ninja Multi-Config instead of Xcode, because we don't support building qt with Xcode itself at the moment.

Note that if Ninja / NMC generator is chosen, we should also not apply build flags like -allowProvisioningUpdates.

Also note that it does seem possible to import a pre-existing ios ninja multi-config build, and then the created kit has ninja as the generator.