gio: Add DTLS interfaces

Add a new GDtlsConnection interface, plus derived GDtlsClientConnection
and GDtlsServerConnection interfaces, for implementing Datagram TLS
support in glib-networking.

A GDtlsConnection is a GDatagramBased, so may be used as a normal
datagram socket, wrapping all datagrams from a base GDatagramBased in
DTLS segments.

Test cases are included in the implementation in glib-networking.

https://bugzilla.gnome.org/show_bug.cgi?id=752240

gio/giotypes.h

@@ -223,6 +223,9 @@ typedef struct _GTcpWrapperConnection                       GTcpWrapperConnectio
  * Since: 2.22
  **/
 typedef struct _GThreadedSocketService                      GThreadedSocketService;
+typedef struct _GDtlsConnection               GDtlsConnection;
+typedef struct _GDtlsClientConnection         GDtlsClientConnection; /* Dummy typedef */
+typedef struct _GDtlsServerConnection         GDtlsServerConnection; /* Dummy typedef */
 typedef struct _GThemedIcon                   GThemedIcon;
 typedef struct _GTlsCertificate               GTlsCertificate;
 typedef struct _GTlsClientConnection          GTlsClientConnection; /* Dummy typedef */