Readdition of old "Open in same location" behavior

Hello! I am a huge fan of gedit, and I have used it exclusively for essentially all my projects. A few years ago, the newer versions of gedit removed one of my favorite features that are standard in code-centeric text editors: opening from the same directory as the currently-opened document. This was essential when working in more complicated projects where simple bookmarks couldn't suffice.

This branch re-adds this feature as the default behavior. I wanted to add it as an option in the preferences to maintain the existing behavior and only do this new behavior when wanted, but I think I would need some assistance in editing the preferences menu. I see the output GLADE XML, but don't think I should be editing it by hand. This would also include the addition of a GSettings string, so I figured I would just implement the basic behavior and discuss it further from there. Since this is a change to the base behavior of gedit, I want to start simple.