support Wayland GTK backend in Window.GetHandle

This adds support for getting a Wayland `wl_surface` from
`Window.GetHandle`. The behavior of the function is now to return either
an X11 window ID or Wayland `wl_surface` if running a GTK build on those
backends, return 0 on any other GTK backend, or else return the C++
`GetHandle` result unmodified. This is the simplest fix for the Wayland
issue without worrying about breaking existing user code, as this
function already failed for any non-X11 GTK build. Returning 0 on
non-X11/Wayland GTK platforms sidesteps the issue of making sure e.g.
GTK Mac GetHandle returns the same type as native Cocoa Mac GetHandle.

etg/window.py

@@ -136,6 +136,16 @@ def run():
     m1.find('externalLeading').out = True
 
     c.find('GetHandle').type = 'wxUIntPtr*'
+    c.find('GetHandle').detailedDoc = [
+        """The returned value differs from the C++ version of GetHandle when \
+        running on the GTK port. When running on Wayland with GTK, this \
+        function returns a `wl_surface` pointer for the native OS window \
+        containing the widget. On X11 with GTK, this returns the X window \
+        ID for the containing window. On any other backend with GTK, this \
+        function returns 0.\n\n"""
+
+        """On some platforms this may return 0 if the window has not yet been shown."""
+    ]
     c.find('GetHandle').setCppCode("return new wxUIntPtr(wxPyGetWinHandle(self));")
 
     c.addCppMethod('void*', 'GetGtkWidget', '()', """\

src/window_ex.cpp

@@ -1,32 +1,15 @@
-
-#ifdef __WXMSW__
-#include <wx/msw/private.h>
-#endif
-
 #ifdef __WXGTK__
-#include <gdk/gdkx.h>
+#include <gdk/gdk.h>
 #include <gtk/gtk.h>
-#ifdef __WXGTK3__
-// Unlike GDK_WINDOW_XWINDOW, GDK_WINDOW_XID can't handle a NULL, so check 1st
-static XID GetXWindow(const wxWindow* wxwin) {
-    if ((wxwin)->m_wxwindow) {
-        if (gtk_widget_get_window((wxwin)->m_wxwindow))
-            return GDK_WINDOW_XID(gtk_widget_get_window((wxwin)->m_wxwindow));
-        return 0;
-    }
-    else {
-        if (gtk_widget_get_window((wxwin)->m_widget))
-            return GDK_WINDOW_XID(gtk_widget_get_window((wxwin)->m_widget));
-        return 0;
-    }
-}
-#else
-#define GetXWindow(wxwin) (wxwin)->m_wxwindow ? \
-                          GDK_WINDOW_XWINDOW((wxwin)->m_wxwindow->window) : \
-                          GDK_WINDOW_XWINDOW((wxwin)->m_widget->window)
+
+#ifdef GDK_WINDOWING_X11
+#include <gdk/gdkx.h>
 #endif
+#ifdef GDK_WINDOWING_WAYLAND
+#include <gdk/gdkwayland.h>
 #endif
 
+#endif // ifdef __WXGTK__
 
 
 
@@ -36,11 +19,40 @@ wxUIntPtr wxPyGetWinHandle(const wxWindow* win)
 #ifdef __WXMSW__
     return (wxUIntPtr)win->GetHandle();
 #endif
-#if defined(__WXGTK__) || defined(__WXX11__)
-    return (wxUIntPtr)GetXWindow(win);
+
+#ifdef __WXX11__
+    return (wxUIntPtr)win->GetHandle();
 #endif
+
 #ifdef __WXMAC__
     return (wxUIntPtr)win->GetHandle();
 #endif
+
+#ifdef __WXGTK__
+    GtkWidget *gtk_widget = win->GetHandle();
+    if (!gtk_widget) {
+        return 0;
+    };
+    // gtk_widget_get_window disappears in GTK4; then it will be via
+    // gtk_widget_get_native() -> gtk_native_get_surface().
+    GdkWindow *window = gtk_widget_get_window(gtk_widget);
+    if (!window) {
+        return 0;
+    }
+#ifdef GDK_WINDOWING_X11
+    if (GDK_IS_X11_WINDOW(window)) {
+        return (wxUIntPtr)gdk_x11_window_get_xid(window);
+    }
+#endif
+#ifdef GDK_WINDOWING_WAYLAND
+    if (GDK_IS_WAYLAND_WINDOW(window)) {
+        return (wxUIntPtr)gdk_wayland_window_get_wl_surface(window);
+    }
+#endif
+    // Returning 0 on any other backend using GTK.
+    // This is less confusing than returning something else that might
+    // mismatch C++ GetHandle.
+#endif
+
     return 0;
 }