Add G_SIGNAL_DEPRECATED

Similar to G_PARAM_DEPRECATED. It will warn only for users of the
signals, so a signal can still be emited without warning, for
compatibility reasons.

Apparently, there is no way user flags could have been used before,
so that shouldn't break anyone.

https://bugzilla.gnome.org/show_bug.cgi?id=663581

gobject/gsignal.h

@@ -110,6 +110,9 @@ typedef gboolean (*GSignalAccumulator)	(GSignalInvocationHint *ihint,
  * @G_SIGNAL_NO_HOOKS: No emissions hooks are supported for this signal.
  * @G_SIGNAL_MUST_COLLECT: Varargs signal emission will always collect the
  *   arguments, even if there are no signal handlers connected.  Since 2.30.
+ * @G_SIGNAL_DEPRECATED: The signal is deprecated and will be removed
+ *   in a future version. A warning will be generated if it is connected while
+ *   running with G_ENABLE_DIAGNOSTIC=1.  Since 2.32.
  * 
  * The signal flags are used to specify a signal's behaviour, the overall
  * signal description outlines how especially the RUN flags control the
@@ -124,14 +127,15 @@ typedef enum
   G_SIGNAL_DETAILED	= 1 << 4,
   G_SIGNAL_ACTION	= 1 << 5,
   G_SIGNAL_NO_HOOKS	= 1 << 6,
-  G_SIGNAL_MUST_COLLECT = 1 << 7
+  G_SIGNAL_MUST_COLLECT = 1 << 7,
+  G_SIGNAL_DEPRECATED   = 1 << 8,
 } GSignalFlags;
 /**
  * G_SIGNAL_FLAGS_MASK:
  * 
  * A mask for all #GSignalFlags bits.
  */
-#define G_SIGNAL_FLAGS_MASK  0xff
+#define G_SIGNAL_FLAGS_MASK  0x1ff
 /**
  * GConnectFlags:
  * @G_CONNECT_AFTER: whether the handler should be called before or after the