Add embedable images (GraphiteEditor#564)

* Add embedable bitmaps

* Initial work on blob urls

* Finish implementing data url

* Fix some bugs

* Rename bitmap to image

* Fix loading image on document load

* Add transform properties for image

* Remove some logging

* Add image dimensions

* Implement system copy and paste

* Fix pasting images

* Fix test

* Address code review

Co-authored-by: Keavon Chambers <keavon@keavon.com>