Implement wxNotificationMessage using libnotify in wxGTK.

Use libnotify -- if detected by configure -- to provide native notifications
in wxGTK. Our API maps to libnotify one in rather straightforward way, we
might consider extending it to cover more of libnotify functionality
(categories, user-defined icons and, especially, actions) later.

Also update the dialogs sample to show another kind of notification and the
documentation to clarify the behaviour of various methods.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72218 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

setup.h_vms

@@ -869,6 +869,10 @@ typedef pid_t GPid;
  * Use the Hildon 2.0 framework
  */
 #define wxUSE_LIBHILDON2 0
+/*
+ * Use libnotify library.
+ */
+#define wxUSE_LIBNOTIFY 0
 
 #define wxUSE_DC_TRANSFORM_MATRIX 1