Slideshow timed

Add an option for timings on the slideshow.  The user can set the timings and then have the slideshow play back to them.... without clicking the left and right buttons.  This is laying the ground work for a video export feature.  I will hopefully sent a pull request for that, but it might be a bit slow because gst is involved :)

UI design for the set duration feature:

![screenshot from 2014-12-25 17 26 16](https://cloud.githubusercontent.com/assets/6022042/5552645/301d554e-8c5b-11e4-86a2-dfbb0079e94d.png)

I'm not sure what the GNOME icon is about :)

That is about all the UI changes (besides the *very complex* removal of the buttons on the slideshow mode when the user is in the timed slideshow)

Have a nice day :santa: :christmas_tree: