wxGTK: Add private options to bring back traditional scrollbar(always…

… visible).

GTK3 defaults to enable the 'overlay-scrolling' features, that is, only show scrollbar when the mouse/touching
hover the scrollbar area. It is undesirable to many of our clients.
Yet wxWidgets does NOT exposing any API for us to do the GTK customization.
So we need to add our 'CMExtension' to workaround this problem.

This MAY be a temporary solution. We may make a new patch in the future by introducing a new properly designed
API to wxWidgets for this sort of native/per-platform customization.

include/wx/window.h

@@ -49,6 +49,12 @@
     #define wxUSE_MENUS_NATIVE wxUSE_MENUS
 #endif // __WXUNIVERSAL__/!__WXUNIVERSAL__
 
+#ifdef __WXGTK__
+namespace CMExtension {
+    extern bool g_need_traditional_scrollbar;
+}
+#endif
+
 // ----------------------------------------------------------------------------
 // forward declarations
 // ----------------------------------------------------------------------------

src/gtk/window.cpp

@@ -72,6 +72,10 @@ typedef guint KeySym;
     #define PANGO_VERSION_CHECK(a,b,c) 0
 #endif
 
+namespace CMExtension {
+    bool g_need_traditional_scrollbar = false;
+}
+
 //-----------------------------------------------------------------------------
 // documentation on internals
 //-----------------------------------------------------------------------------
@@ -2764,6 +2768,9 @@ void wxWindowGTK::GTKCreateScrolledWindowWith(GtkWidget* view)
     m_widget = gtk_scrolled_window_new( NULL, NULL );
 
     GtkScrolledWindow *scrolledWindow = GTK_SCROLLED_WINDOW(m_widget);
+
+    if (CMExtension::g_need_traditional_scrollbar)
+        g_object_set(scrolledWindow, "overlay-scrolling", false, NULL);
 
     // There is a conflict with default bindings at GTK+
     // level between scrolled windows and notebooks both of which want to use