Add Natural Sorting for thumbnail view.

This addresses bug #717960, "Natural sorting of photo titles"
I grafted NaturalSort.vala from my own https://github.com/tobiatesan/NaturalCmp

Please find the ticket at: https://bugzilla.gnome.org/show_bug.cgi?id=717960
Fix small typo in comment for NaturalCmp.compare
Replace if()s with asserts() in NaturalCmp