This commit adds support for uploading images to i.nuuls.com from clipboard or by dragging an image into a split.
Documentation for how to self-host the image uploader is available in ENV.md
By default, you will be asked before an image upload takes place. There's an option in the dialog to not be asked again, if that option is chosen you can revert that choice in the settings dialog.
Fixes#1546
This change introduces a "window timer" that runs every 100ms that we
use to update the pixmap if necessary, since there is no signal for "let
me know when this image is done loading".