Clean up m_resource_map after calling destroy_resource() func

Currently, when a resource gets destroyed, the destroy func will be
called after the resource is removed from the resource map, which seems
pretty reasonable!

On the other hand, if the wrapper operates in the multi-cast mode, we
want m_resource to be valid when the destroy_resource() function is
called to properly perform cleanup.

Another issue is that given that m_resource_map is cleaned up before
calling the destroy_resource() function, the object pointer of the
resource won't be cleaned up if the wrapper is destroyed inside the
destroy_resource() function.

In order to make the destroy listener code more comprehensible, this
change makes the generated code clean up m_resource_map **after**
invoking the destroy_resource() function. There are a couple of reasons

* Unify the behavior of destroy_resource() for unicast and multicast
  wrappers
* It makes the resourceMap() API consistent with libwayland-server API,
  in particular wl_client_for_each_resource(). When the resource destroy
  listener is invoked, wl_client_for_each_resource() won't exclude the
  destroyed resource

Unfortunately, it is also an API breaking change. You cannot have code
something like

    void Foobar::zwp_foobar_destroy_resource(Resource *resource)
    {
        if (resourceMap().isEmpty()) {
            delete this;
        }
    }

Instead, you should explicitly destroy inert wrappers, which is probably
a good thing.
1 job for !173 with work/fix-asan in 7 minutes and 27 seconds (queued for 23 minutes and 7 seconds)
latest detached
Status Name Job ID Coverage
  Build
passed linux kf5-qt5 qt5.15 #92921

00:07:27

66.0%