Better support for controllers without stop button

[Benjamin Klum]

Let's assume record-if-slot-empty mode and exclusive mode are enabled and the user presses a MIDI pad of a playing slot. Now, if the slot's group has a MIDI trigger assigned (for stopping it), Playtime will retrigger the clip - as it does now. But if not, it will stop the clip, because it knows the user probably wants to have a way to stop clips. See http://forum.cockos.com/showpost.php?p=1561396&postcount=442

[Benjamin Klum]

re #164 When pad is pressed if clip is playing and group has no MIDI trigger assigned and MIDI toggle mode, exclusive mode and record-if-slot-empty mode are enabled, don't retrigger but stop

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