Fix wxGTK build with glib < 2.32 and streamline the code a bit

Define g_signal_handlers_disconnect_by_data() if it's not available,
i.e. when using glib older than 2.32 where it was added, to fix the
build under old systems such as CentOS 6 broken by the changes of
8278f7b618 (see #18084).

Also use it elsewhere instead of g_signal_handlers_disconnect_matched()
as it's more readable.

Closes https://github.com/wxWidgets/wxWidgets/pull/760

include/wx/gtk/private/gtk2-compat.h

@@ -425,6 +425,14 @@ static inline void wx_gdk_cairo_set_source_window(cairo_t* cr, GdkWindow* window
 #define gdk_cairo_set_source_window wx_gdk_cairo_set_source_window
 #endif
 
+// ----------------------------------------------------------------------------
+// the following were introduced in Glib 2.32
+
+#ifndef g_signal_handlers_disconnect_by_data
+#define g_signal_handlers_disconnect_by_data(instance, data) \
+      g_signal_handlers_disconnect_matched ((instance), G_SIGNAL_MATCH_DATA, 0, 0, NULL, NULL, (data))
+#endif
+
 // ----------------------------------------------------------------------------
 // the following were introduced in GTK+ 3.0