Add scale property

Add scale property to calculate the difference between the OSD
and the base application.
use secs from parameter
Check for the stereo attribute before using it
Add status property to get information about transitions
Clip photo widget to its size
Add some properties for the GUI
Add TOGGLE_OSD event support

The GUI now handles the TOGGLE_OSD for images inside the theme
code without the actual viewer knowing about this. The code works
nice but needs some cleanup.