core: prevent early MetaIdleMonitor destruction when its invoker vanishes

If the last reference of a MetaIdleMonitor is held by the caller, it may
happen that the last reference is lost when calling the GDestroyNotify,
if this happens when the watched DBus name vanishes, the object (and the
watches hashtable) are destroyed while manipulating the watches hashtable,
so bad things may happen then.

Fix this by wrapping the operation by a ref/unref pair, so the object would
be destroyed after operating on the hashtable.

https://bugzilla.gnome.org/show_bug.cgi?id=724969