More adjustments to focus

Previous change is not enough, as WM_KILLFOCUS can apparently be sent to
window losing focus after WM_ACTIVATE has been sent to window gaining focus

Try using WM_SETFOCUS instead, as that has the correct ordering and seems
more logical.

The test "!pWin || !pWin->overrideRedirect" is confusingly written: It's
true if:

(a) pWin is NULL (= X window doesn't exist, shouldn't happen), or
(b) pWin->overrideRedirect is FALSE

i.e. the intended effect is "don't give focus to override redirect windows"

XXX: ignoring WM_ACTIVATE doesn't seem to be always correct:

If you minimize a maximized Windows window, and window activation moves
to a maximized X window beneath it, the X input focus doesn't move...
6 jobs for focus in 8 minutes and 56 seconds (queued for 7 seconds)
latest
Status Name Job ID Coverage
  Docker Image
passed debian-buster #13129834

00:00:58

 
  Build And Test
passed autotools #13129835

00:07:57

passed meson #13129836

00:03:26

passed meson-noglamor #13129837

00:02:44

passed mingw-cross-build #13129838

00:00:56

 
  External
failed continuous-integration/appveyor/branch