decodebin3: Avoid selection recalculation

Currently the way decodebin3 figures out whether it should modify the current selection is not reliable.

What it should do, after posting a new collection is to check whether there already was a selection requested for that collection.

The order of events is:

• Updated collection is stored as dbin->collection
• Collection is posted on the bus
• update_requested_selection is called

Three things should happen:

• When the dbin->collection is modified, dbin->select_streams_seqnum should be resetted
• UNCHANGED : If a select_streams event comes in, we store it along with the seqnum
• Within update_requested_selection, and with the SELECTION lock taken, Only calculate a new selection IFF the seqnum is not set, otherwise just use the selection.