launch: handle PROGRESS messages

launch: handle PROGRESS messages

Wait for all PROGRESS messages (if any) to complete before going to the PLAYING
state. This is the only way we can wait for live elements to complete their
operations.

This is interesting for elements like rtspsrc that do some asynchronous network
requests as part of going to the PAUSED state. It could be possible that it, for
example, provides a clock and then we would like to wait until it completes
so that we can use the provided clock when going to PLAYING.