This commit was created using a script that searched for all docstrings
containing a parameter and the string 'or %NULL'.
Gdk backends and demos excluded as they are not part of a public API
https://bugzilla.gnome.org/show_bug.cgi?id=610474
Adds a 'model' type buffer for GtkEntry in which the actual
textual data is stored. GtkEntryBuffer can be subclassed.
Among other things, this allows GtkEntry to be used for secrets
that need to be stored in non-pageable memory. It also allows
buffers to be shared by entries.
See bug #576801.
