Consider changing qt-cmake to use the Xcode generator when building Qt for iOS

All of our qt-cmake shell wrappers specify no CMake generator to use, to allow the developer to choose which one they want.

The usual makefiles / ninja generator is not really usable on iOS. Just for iOS, consider defaulting to the Xcode generator when using qt-cmake.
Need to check if this breaks anything in Qt Creator integration.
And ideally the script wrapper should be smart enough to allow specifying a different generator (so check to only specify the var if it wasn't specified by the user on the command line)