handle mouse events

def on_mouse_event(self, event):
        '''handle mouse events'''
        pos = event.GetPosition()
        if event.RightDown() and self.popup_menu is not None:
            self.show_popup_menu(pos)
            return
        if event.Leaving():
            self.mouse_pos = None
        else:
            self.mouse_pos = pos

        if event.LeftDown():
            self.mouse_down = self.image_coordinates(pos)
        if event.Dragging() and event.ButtonIsDown(wx.MOUSE_BTN_LEFT):
            self.on_drag_event(event)