[server] Introduce a Resource::unbound signal emitted from unbind handler

Summary:
So far for internal cleanup we mostly listen to QObject::destroyed.
When a Resource gets unbind the wl_resource is set to null and the
Resource gets deleteLater. This creates a short time frame when the
Resource is still there, but the wl_resource is null. For most internal
usages the Resource is completely useless at that point and should no
longer be considered. So far it was still considered and could hit
crashers, if a code path did not nullptr check. Unfortunately
libwayland-server is not nullptr safe: if called with a null value it
tends to crash.

So this check introduces a new signal unbound which can be listend to
in addition to the destroyed signal. It's used in SeatInterface for
DataDeviceInterface, where we experienced a crash related to that.

A test case is added which exposes the crash, but it already needs
the unbound signal to get into the crashy condition. The actual crash
is fixed twice - with the help of the unbound signal, but also by
introducing the nullptr check where it's needed.

Reviewers: #plasma_on_wayland

Subscribers: plasma-devel

Tags: #plasma_on_wayland

Differential Revision: https://phabricator.kde.org/D1868