Port to GtkSourceFileSaver
Remove document-loader and document-saver

And their contained classes: document-input-stream and
document-output-stream.

All this code is now in GtkSourceView.
GeditDocument: remove unused code
GeditDocument: make set_readonly() static
Port to GtkSourceCompressionType

And remove GeditDocumentCompressionType. This is an API break, but the
port is trivial.