- Stop arbitrarily consider params as ClockTime based on their names
but add a convetion that the `.type` field of the ActionType should
end by `(GstClockTime)` when it is a clock time.
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.
