Make the following functions private:
gedit_document_load
gedit_document_load_stream
gedit_document_load_cancel
gedit_document_save
gedit_document_save_as

They are normally not needed by plugins. What plugins need is to do some
stuff before/after a file loading or saving, i.e. the following signals
are needed: "load", "loaded", "save", "saved".

By making the above functions private, it'll be easier to port
GeditDocument and GeditTab to GtkSourceFile.