Update File: Open functionality

Will now open in a new ImageCraft instance if we have work done in the
current one, otherwise it opens as before in the current ImageCraft.
Currently working on making the drawingArea resize if the image is
bigger than it, but my battery is low so I want to push in case you guys
want to work on it. Feel free to finish that up if you'd like. Otherwise
I'll work on it later.