ffplay: add support for interactive volume control

This is a feature heavily inspired by the mpv player. At the moment, methods
for adjusting volume in ffplay are rather clumsy: either one needs to set it
system-wide, or one needs to set it via the volume filter.

This patch adds key bindings identical to the mpv defaults for muting/unmuting
and increasing/decreasing the volume interactively without any introduction of
external dependencies.

TODO: doc update, possible mouse button bindings (mpv has this).
Signed-off-by: NGanesh Ajjanagadde <gajjanagadde@gmail.com>
Signed-off-by: NMarton Balint <cus@passwd.hu>