src/keyboard.c: do not end window-cycling on modifier release only

src/keyboard.c: do not end window-cycling on modifier release only

If a user lets go of the modifier (e.g. alt) before the 'normal' key (e.g.
tab) when window-cycling, we do not end the cycling until both keys have
been released.  If we end the window-cycling on release of the modifier
only, some XWayland clients such as hexchat realise that tab is pressed
(even though we did not forward the event) and because we absorb the
equivalent release event it gets stuck on repeat.

Just to clarify the position here: Issue #176 describes a behaviour
whereby dmenu gets stuck on repeat after being launched with a keybind.
This patch does not resolve that issue but reflects that in Wayland, the
client is responsible for implementing "key repeat".

Changing the key repeat rate/delay in (labwc/labwc@e62bb51) was dirty fix
that need should never have been made.