File > Import should allow the user to place the image(s) in the viewport

File > Import should support multiple imports at once. If there's no document open, it should do what it does now by creating a document with the correctly sized artboard, one per imported file. Except we should probably make the be part of File > Open instead of Import, so Open works with `.graphite` files as well as regular images.

But if a document is open, that's what Import is for. We want that to not place the import in the center of the viewport's current view, but instead attach it to the user's cursor and show a thumbnail preview of the image. Clicking places it center at the chosen location. Dragging draws a box with the image's aspect ratio which dictates where it will be placed. Once that's complete, it moves on to the next one if multiple images were imported.