Properly handle destroying a Keyboard resource

Summary:
On client side the newer wl_keyboard_release is used which is a
destructor call. On server side the shared destroy callback is used
and it's ensured that KWayland doesn't crash if called into the
KeyboardInterface between unbound and destroyed.

Test Plan:
Test case extended to cover the condition of an unbound
KeyboardInterface.

Reviewers: Plasma on Wayland

Subscribers: plasma-devel

Tags: Plasma on Wayland

Differential Revision: https://phabricator.kde.org/D2036