wayland: selectively cancel key repeat on key release

wayland: selectively cancel key repeat on key release

Under Wayland, when multiple keys are pressed and the user releases a
key, key repeat should continue unless the key released is the one
currently repeating.

In the case of:

- key1 press
- key1 repeat
- key2 press -> key1 repeat stopped
- key2 repeat
- key2 release

The behavior should be to cancel keyboard repeat, though key1 is still
held down. This is consistent with prior X11/XWayland behavior.

The following also must work:

- key1 press
- key2 press
- key2 release
- key2 press
- key1 release
- key2 should continue to repeat

The fix for bug #778019 should continue to work:

- key1 press
- key1 repeat
- key2 press -> key1 repeat stopped
- key1 release
- key2 should repeat

The choice to change the counter nkeys to the flag repeat_active
helps to solve the second test case.

https://bugzilla.gnome.org/show_bug.cgi?id=781285