Unschedule any remaining scheduled items when a button is pressed

This means that less confusion will reign if a button is pressed
at the wrong time, and catch-up needs to happen. Randomly switching
out lyrics/images for things scheduled from the previous tranche
just leads to chaos.

gig.py

@@ -70,6 +70,8 @@ class EventRunner:
             elif marker != 'select':
                 return False
             self._selected = selected
+        else:
+            self.stop(full=False)
         now = time.time()
         if not self._currentitem.activate(marker, now):
             return False
@@ -82,8 +84,11 @@ class EventRunner:
         self._next_button_is_prestart = self._currentitem.is_button_prestart(self._next_button)
         return True
 
-    def stop(self):
+    def stop(self, full=True):
+        print("Unscheduling events")
         pyglet.clock.unschedule(self.update)
+        if not full:
+            return
         self._playing = None
         self._selected = None
         for event in self._active: