make it possible to switch between WinTab and Windows Ink on supported tablets

We really should not consider WinTab to be obsolete. Some applications (e.g. ArtRage and others) have checkboxes in their own settings to control whether to use WinTab or Windows Ink, because Ink has limitations. So it would be nice to make this easy for Qt applications to do.

Windows Ink supports only one button on the stylus; but other buttons come through via mouse events with some “extra data” packet. So Qt should be able to get them, even when using Ink.

Wacom really wants us to support all the extra controls on the tablet itself too (sliders, rings, buttons etc.) On Windows and macOS, the traditional way is to install the drivers and use the control panel to map those to other kinds of events (keystrokes, mouse buttons, wheel or whatever); but that’s because of limitations with OS APIs. But with WinTab we can get direct access to those. On Linux, of course there’s no standard control panel for mapping that stuff, and Peter Hutterer the libinput guy doesn’t believe in that idea at all; so to have feature parity between Linux and the other OSes, we should be able to send events that come from the raw valuators directly from the tablet. So we should continue supporting WinTab, because that’s the only way to get that stuff. We could either prefer WinTab if it’s available, and fall back to Windows Ink; and/or we could offer some way that applications can switch at runtime (to support the ones that want to have checkboxes in their own settings, like ArtRage does).

Wacom folks were also thinking for a while that WinTab would fade away, but not anymore; they will continue to support it for the foreseeable future. But they never send events from the same tablet via both APIs: the application has to choose. If you use a Microsoft Surface or the the like, of course installing Wacom drivers isn’t necessary, so Windows Ink is the only way that will work. But if you connect an Intuos tablet to the same machine, and install Wacom drivers, then WinTab becomes available for both the internal tablet and the external one, and now the application can choose whether to use WinTab or Ink. So Qt should make it possible for applications to choose, ideally.