Multithreading for loading Animated Gifs, ...

- The fetching of animated gif frames are now done in a separate thread.
This means they are displayed much sooner. If the animated gif is large
the animation may not be very smooth until it loops.
- All the options should now be bindable to both keyboard keys and mouse
buttons.
- Fixed Animated Gifs always reloading when going back to them when
navigating with the next/prev image keys.