glib/mem: Add g_clear_pointer_with() utility

Countless of times I've ended up having the following code pattern:

g_clear_object (&self->some_object);
g_clear_pointer (&self->data, g_free);
g_clear_pointer (&self->blob, blob_free);

if (self->borrowed_thing)
  {
    actual_owner_remove (self->actual_owner, self->borrowed_thing);
    self->borrowed_thing = NULL;
  }

if (self->borrowed_other_thing)
  {
    actual_owner_remove (self->actual_owner, self->borrowed_other_thing);
    self->borrowed_other_thing = NULL;
  }

Which is annoying beacuse in no way are the last 10 lines more relevant than the first three. This new utility function makes it possible to change the above to:

g_clear_object (&self->some_object);
g_clear_pointer (&self->data, g_free);
g_clear_pointer (&self->blob, blob_free);
g_clear_pointer_with (&self->borrowed_thing, self->actual_owner,
                      actual_owner_remove);
g_clear_pointer_with (&self->borrowed_other_thing, self->actual_owner,
                      actual_owner_remove);

Which makes it less of a punishment to deal with objects that needs to be removed from somewhere else instead of being just destroyed or freed.

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Got annoyed by yet again have to write all the "if not null remove and set to null" boiler plate in a dispose function, so wrote this as a temporary relief. Any interesting in actually adding?