PsyStimulus: differentiate between a scheduled state and a playing state and timed onset when playing and finishing

[maartenuni]

Currently, when playing a stimulus the [[email protected]_started] and [[email protected]_finished] are called when the stimulus
is about to start. This actually reflects that the stimulus is scheduled by the the PsyAudioDevice or With the PsyCanvas, in case of PsyAuditoryStimulus and PsyVisualStimulus respectively.

So the psy_stimulus_set_is_started and psy_stimulus_set_is_finished are actually (at least they should), prior to the stimulus is actually playing, or stopping so it looks like:

stim_scheduled | ---------****************-------
stim_playing   | ------------**************------
               __ time -> ______________________

Hence, the terminology isn't perfect, perhaps it should be psy_stimulus_set_is_scheduled(True/False). The current set_is_playing could be complemented with a set_is_scheduled, from there we can add a PsyTimer function where we set a timeout on the time stimulus actually starts playing and we set the StimulusPrivate:playing to true. The timer can add an event to the Main MainContext when the timer expires, similarly when unscheduled a stimulus is unscheduled we can set a timer when we expect it to stop.