Add accessors for the type/sdp fields of the WebRTCSessionDescription

See https://github.com/sdroege/gstreamer-rs/issues/108#issuecomment-393505826

gstreamer-webrtc/src/web_rtc_session_description.rs

@@ -25,4 +25,12 @@ impl WebRTCSessionDescription {
             desc
         }
     }
+
+    pub fn get_type(&self) -> ::WebRTCSDPType {
+        unsafe { from_glib((*self.to_glib_none().0).type_) }
+    }
+
+    pub fn get_sdp(&self) -> gst_sdp::SDPMessage {
+        unsafe { from_glib_none((*self.to_glib_none().0).sdp) }
+    }
 }