fallbackswitch: add 'stop-on-eos' property
Fix the following use case:
- main input of fallbackswitch is finite (a media file)
- fallback input is infinite (videotestsrc)
- main input is done and send eos, which is propagated downstream
- fallbackswitch switches to fallback, sending STREAM_START which reset EOS downstream (aggregator does that)
- fallback input keeps pushing buffers forever.
Solve it by adding a 'stop-on-eos' property so fallbackswitch stops pushing property once the main input is eos.