[Windows] Missing keyboard navigation code

[tim-weis]

winit currently doesn't support keyboard navigation in windows (e.g. TAB navigation). Windows' window manager implements this already (see IsDialogMessageW), but winit doesn't employ it.

It would be great to get support for keyboard navigation in winit out of the box. To get there, all message-retrieval code would need to be revisited and updated to either:

• include a call to IsDialogMessageW, or
• arrange for a user-supplied callback, so users can opt-in if they desire.

I acknowledge that neither of these suggestions is trivial to implement, with the latter likely opening up a whole new can of worms.

The following links by Raymond Chen may prove to be helpful in evaluating cost and risk for implementing the former:

• Using the TAB key to navigate in non-dialogs
• Using the TAB key to navigate in non-dialogs, redux

[Osspial]

I don't think it's Winit's place to do this, since there isn't a clear simple API for the feature and (as you said) it's non-trivial to implement. However, once #1508 is merged it should be possible to implement this outside of Winit's codebase.