Add G_SIGNAL_MUST_COLLECT

In some cases, signal arguments have to be collected, even if there are i
no signal handlers connected (e.g. for GVariant parameters, where collection
consumes a floating variant).

Based on a patch by Christian Persch.

Bug #643624.

gobject/gsignal.h

@@ -108,6 +108,8 @@ typedef gboolean (*GSignalAccumulator)	(GSignalInvocationHint *ihint,
  *  of as object methods which can be called generically by 
  *  third-party code.
  * @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.
  * 
  * The signal flags are used to specify a signal's behaviour, the overall
  * signal description outlines how especially the RUN flags control the
@@ -121,14 +123,15 @@ typedef enum
   G_SIGNAL_NO_RECURSE	= 1 << 3,
   G_SIGNAL_DETAILED	= 1 << 4,
   G_SIGNAL_ACTION	= 1 << 5,
-  G_SIGNAL_NO_HOOKS	= 1 << 6
+  G_SIGNAL_NO_HOOKS	= 1 << 6,
+  G_SIGNAL_MUST_COLLECT = 1 << 7
 } GSignalFlags;
 /**
  * G_SIGNAL_FLAGS_MASK:
  * 
  * A mask for all #GSignalFlags bits.
  */
-#define G_SIGNAL_FLAGS_MASK  0x7f
+#define G_SIGNAL_FLAGS_MASK  0xff
 /**
  * GConnectFlags:
  * @G_CONNECT_AFTER: whether the handler should be called before or after the