subtitleoverlay: Don't block on non-serialized events

https://bugzilla.gnome.org/show_bug.cgi?id=729321
playsinkconvertbin: Don't block on non-serialized events

https://bugzilla.gnome.org/show_bug.cgi?id=729321
playsink: Don't block on non-serialized events

https://bugzilla.gnome.org/show_bug.cgi?id=729321
playbin: Don't block on non-serialized events

https://bugzilla.gnome.org/show_bug.cgi?id=729321
audio: map channels=1,channel-mask=0 to MONO instead of NONE

Fixes problem in audioconvert, which would end up using
a mixmatrix when converting between different mono format
because it thinks MONO positioning is different from
unpositioned channels, which is not the case in this
special case. The mixmatrix would end up being 0.0 so
audioconvert would convert to silence samples.

https://bugzilla.gnome.org/show_bug.cgi?id=724509