GeditDocument: remove the "compression-type" property

The property has been migrated to GtkSourceFile.
GeditDocument: remove the "newline-type" property

The property has been migrated to GtkSourceFile.
GeditDocument: remove the "encoding" property

The property has been moved to GtkSourceFile.
GeditDocument: add a TODO comment
GeditDocument: remove the "location" property

The property has been moved to GtkSourceFile.