gesture: Ensure late gestures handling the sequence get the group state

It might happen that a gesture claims a sequence before any other gesture
in its group even handled a single event from that sequence. In that case,
ensure the state is set accordingly right when the sequence is handled in
those.

The "group" gesture testcase has been updated to observe this behavior.

testsuite/gtk/gestures.c

@@ -497,8 +497,8 @@ test_group (void)
                    "b1 state denied, "
                    "a1 state denied, "
                    "c3 state claimed, "
+                   "c2 state claimed, "
                    "target c2");
-  /* FIXME: why no "c2 state claimed" ? */
 
   g_string_free (str, TRUE);