GEDIT_DOCUMENT_ERROR_CANT_CREATE_BACKUP is unused

So to simplify the code we can remove its support.
G_IO_ERROR_CANT_CREATE_BACKUP is used instead.
GeditDocument: remove dead code
Deprecate GeditEncoding (use GtkSourceEncoding instead)

GeditEncoding now uses internally GtkSourceEncoding.
Adapter between GtkSourceFile and file saving gedit API