Define prestart attribute on ButtonMark to place buttons before start

timings.py

@@ -17,6 +17,7 @@ class Timings:
         self._markers = {}
         self._marker_objects = {}
         self._marker_buttons = []
+        self._marker_buttons_prestart = {}
         self._tempo_objects = []
         self._tempo = []
         self._resolved = False
@@ -36,6 +37,12 @@ class Timings:
                 return button
         return None
 
+    def is_button_prestart(self, name):
+        if name in self._marker_buttons_prestart:
+            return True
+
+        return False
+
     def add_timing(self, timing):
         self._timings.append(timing)
         self._resolved = False
@@ -73,10 +80,12 @@ class Timings:
         #print("beat_at({}) = {}".format(at, 60 / tempo))
         return 60 / tempo
 
-    def register_mark(self, name, at):
+    def register_mark(self, name, at, prestart=False):
         self._marker_objects[name] = at
         if at is None:
             self._marker_buttons.append(name)
+        if prestart:
+            self._marker_buttons_prestart[name] = True
         self._resolved = False
 
     def register_tempo(self, tempo, beats, at):