Make the DocumentSaver independent of Document

The last dependency was _gedit_document_create_mount_operation().
The mount operation is set to the DocumentSaver, with
gedit_document_saver_set_mount_operation_factory().

So the mount operation comes from GeditViewFrame (to have the parent
window), it is set to GeditDocument with
_gedit_document_set_mount_operation_factory(). Then it is set to
GeditDocumentSaver with a similar function.
DocumentSaver: remove dependency to GeditDocument

A GtkTextBuffer is now sufficient.

gedit-document.h is still needed for GeditMountOperationFactory.