Add support for WatchOS projects

Previously Qt had support for WatchOS by running Qt apps directly on the watch which was not optimal due to the large Qt size and specific UI limitations of WatchOS. Maybe the support for WatchOS was a bit too early. But now I am seeing a significant amount of adaption of smartwatches (in other words, an increasing number of customers ask for WatchOS and I use it myself a lot).

At the moment the integration of WatchOS (WatchKit) with Qt is very cumbersome because the cmake XCode project generation process recreates the XCode project each time.

For a future Qt version it should be possible to generate XCode projects with WatchKit support. This means that Qt prepares the project to include the two WatchOS specific app targets (extensions, see https://developer.apple.com/documentation/watchkit/setting_up_a_watchos_project?language=objc#3295999). Everything running on the watch is written using native WatchOS techniques (i.e. Swift). The watch communicates via WatchKit with the Qt app running on iOS.