Make B flip apps that don't support rotate

ie button_b now always has a default action, if not overridden by the
app.

modules/app.py

@@ -30,7 +30,8 @@ def on_start(self):
             self.buttons.on_press(tidal.BUTTON_FRONT, self.navigate_back)
             if self.supports_rotation():
                 self.buttons.on_press(tidal.BUTTON_B, self.rotate)
-
+            else:
+                self.buttons.on_press(tidal.BUTTON_B, self.flip)
 
     # Note: we don't actually stop apps yet...
     # def on_stop(self):
@@ -148,6 +149,10 @@ def set_rotation(self, rotation, redraw=True):
     def rotate(self):
         self.set_rotation((self.get_rotation() + 90) % 360)
 
+    def flip(self):
+        self.set_rotation((self.get_rotation() + 180) % 360)
+
+
 class ButtonOnlyWindow:
     """This class only exists to wrap a Buttons instance for any App which doesn't actually use a Window for drawing
        (and presumably draws directly to the display in its on_activate)