Refactor the tons of copypasted code between screenshooter-imgur.c and screenshooter-zimagez.c

- move common callbacks to screenshooter-job-callbacks.c
- only pass screenshot name and title to screenshooter_upload_to_imgur
- set a jobtype property on the job gobject to figure out if we need to show imgur or zimagez urls
- add a create_throbber_dialog function to build the dialog displayed during upload

Finally, screenshooter-imgur.c and screenshooter-zimagez.c only have the upload-related code.