a11y: Do not emit window:activate events on temporary grab

When pressing e.g. a window manager shortcut, Xorg would send FocusOut
NotifyGrab then FocusIn NotifyUngrab.  Currently gtk would then send
window:activate/deactivate, which makes the screen reader think that we
have switched to a non-accessible application and came back again, and thus
reannounce the application frame etc. which we don't want when e.g. just
raising volume.

This adds a "grab" field to GdkFocusEvent, so that listeners can know when
focus events are solely due to temporary grabbing, and let window_focus
avoid sending window:activate/deactivate in that case.

Fixes #85
1 job for window-activate-grab in 7 minutes and 51 seconds (queued for 3 seconds)
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passed build #17515

00:07:51