Support tab navigation under Windows

[lpintes]

I just tested the example program from documentation (doc.go), and found that Tab key is completely ignored.
Thus keyboard navigation is not possible under Windows.

[andlabs]

I actually didn't quite understand how tab stops work in Windows: I thought it was just WS_TABSTOP, which all controls do have... though now I know it also has to do with IsDialogMessage(). I can add that later today and see what happens.

Tab /order/ and order customization, on the other hand, is something I'm not sure how to do in any of the backends, though I wonder if that will really be much of an issue.

[andlabs]

Fixed; thanks.

The commit also disables tab stops on labels and progress bars; forgot to mention that in the commit message. Oops.

[andlabs]

Reopening as a reminder to myself to readd this to the rewrite.

It technically works but

• the tab order is messed up
• focus is not preserved across window changes

[andlabs]

Everything works properly now in libui.