And require them to follow the `$varname` (can't be $(varname) as
parenthesis have another meaning in those expressions).
Still accept "duration" and "position" as varname for backward compat
but update our scenarios anyway.
This way it is clear that you are using a variable reading the scenario
and we can verify that what the scenario writer intents is to use an
already set variable.
Right now reverse playback happens till the beginning of the media file.
But for files which are longer than 150 seconds,
Timeout 'Hard timeout reached: 150 secs' error happens. So we should set the
start time within 150 seconds.
https://bugzilla.gnome.org/show_bug.cgi?id=753216
