Handle multiple attachments of the same image #34

Currently, if you attach the same image twice then the ComposeImageManager.set_image_progress () function uses (btn.image_path == image_path) to check when it has found the right button and so always hits the first image and updates the status of that.

Twitter allows multiple copies of the same image (and the upload works through Cawbird, if you ignore the incorrect UI feedback). This means that ComposeJob.image_upload_progress needs reworking so that it doesn't just use the local path.