Fix all ScreenManagers sharing the same transition

All ScreenManagers share the same SlideTransition instance by default, which can cause some timing issues and other weirdness (and is more generally a bad idea). This PR fixes the issue.

kivy/uix/screenmanager.py

@@ -906,7 +906,7 @@ class ScreenManager(FloatLayout):
     to None.
     '''
 
-    transition = ObjectProperty(SlideTransition(), baseclass=TransitionBase)
+    transition = ObjectProperty(baseclass=TransitionBase)
     '''Transition object to use for animating the transition from the current
     screen to the next one being shown.
 
@@ -966,6 +966,7 @@ def _get_screen_names(self):
     def __init__(self, **kwargs):
         super(ScreenManager, self).__init__(**kwargs)
         self.fbind('pos', self._update_pos)
+        self.transition = SlideTransition()
 
     def _screen_name_changed(self, screen, name):
         self.property('screen_names').dispatch(self)