Instead of simply waiting for a fixed amount of ms for the next source,
we wait for the most sensible amount of time. This is whatever amount
of time is remaining to play minus a 0.5 seconds safety delta (time
values not precise etc.).
A source for which we have no time or for which the remaining time is
< 0.5 seconds is immediately unlocked again. Otherwise the consumer has
as much time as gst gave us to set a new source.
This in particular prevents pointless excessive locking on sources which
have a totalTime < whatever fixed lock value we have (so that we'd lock
longer than what we are playing). An issue apparent with notification-like
sounds, that are rather short and do not need gapless transitioning.

BUG: 307530
FIXED-IN: 4.6.3

Conflicts:
	gstreamer/mediaobject.cpp