When a window is not visible we cannot capture its thumbnail. We used to
store the thumbnail when a window was visible so that we could reuse it
when minimized, but this can cause visual glitches.

When creating a new launcher, we try and find an available object ID by
browsing the existing ones.  This however fails to take into account
objects that were created but that didn't make it to the backend yet,
leading to likely re-using the same ID when creating several objects
roughly at the same time.  This happens e.g. when dropping several URIs
together.

Let the GError ownership on the async finish method call.