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

src/gtk/window.cpp

@@ -2675,8 +2675,7 @@ bool wxWindowGTK::Create( wxWindow *parent,
 
 void wxWindowGTK::GTKDisconnect(void* instance)
 {
-    g_signal_handlers_disconnect_matched(instance,
-        GSignalMatchType(G_SIGNAL_MATCH_DATA), 0, 0, NULL, NULL, this);
+    g_signal_handlers_disconnect_by_data(instance, this);
 }
 
 wxWindowGTK::~wxWindowGTK()