Adding GstAggregator based input-selector-like elements. streamselector element behaves similar to input-selector element, but active pad selection is done via "active" property of streamselector's sink pads. And input streams will be synchronized by using active pad's segment (same as "sync-mode=active-segment" of input-selector). In addition to streamselector element, streamselectorbin is added which consists of clocksync and streamselector elements. The streamselectorbin supports "sync-mode" property identical to that of input-selector.