A scenario correspond to a suite of action to execute on a pipeline,
for the time being, we only support seeking the pipeline, but in the
future we can imagine doing some queries, setting pipeline state, etc...
The scenario can be loaded thanks to the GST_QA_SCENARIO environment
variable, making it usable with any existant application, in case, the
application can be used interactively, the user should either, not load
any scenario or let the application run without interacting with it.
The base class adds a 'object' property to hold the monitored object,
it can only be set on construction. Also the constructor now
automatically calls the element set up
