Fix tabbox and xwayland grabbing

wayland: Move XWayland key forwarding into a filter

We optionally send some keys to xwayland through the filter when no x11
client has focus. This allows shortcut handling in X11 apps to work.

When kwin is grabbing keys we don't necessarily want X11 to sniff these
keys as things can get out of sync. A key place is the tabbox. The X11
client sill has focus, but xwayland is not active. This means we pass
tab keys to X which then go to application incorrectly.

Part of this patch changes the tabbox filter to not intercept the alt
key release event. This ensures xwayland's concept of pressed modifiers
stays in sync.


BUG: 484992