Add support for dropping images onto the canvas

This MR lets you grab an image or image file (must be local, for now) and drop it onto the canvas, and have it embedded into your drawing!

I'm a little bit unsure of the sizing of the images, since right now it uses the actual size of the image in view coordinates, that should probably pick a reasonable size (preserving aspect ratio, of course) and let you resize it. Unfortunately, I don't think we have support for resizing anything right now :P