streamselector: Add new input stream selector element

Seungha Yang requested to merge seungha.yang/gstreamer:stream-selector into main
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.
Edited by Seungha Yang

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