Properly handle destroying a Pointer resource

Summary:
On client side the newer wl_pointer_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
PointerInterface between unbound and destroyed.

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

Reviewers: #plasma_on_wayland

Subscribers: plasma-devel

Tags: #plasma_on_wayland

Differential Revision: https://phabricator.kde.org/D2037