Enable Undo/Redo for tempo markers introduced using OSC command

[theGreatWhiteShark]

Right now (051e723) the Event type for indicating changes from the core to the GUI can only provide a single integer. This is AFAIK not enough to provide information of both the location and the speed of a new tempo marker. As soon as we have a more powerful way of communication, the undo/redo of the tempo markers introduced using OSC will be integrated. (But this is most probably not a pressing issue)