Implement wxSTAY_ON_TOP for wxMessageDialog in wxGTK.

Call gtk_window_set_keep_above() to force the message dialog to be above the
other windows (some WMs do this by default anyhow but not all).

Closes #11163.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@61828 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

interface/wx/msgdlg.h

@@ -51,8 +51,8 @@
         @c wxYES_NO is not given so it is usually unnecessary to specify it
         explicitly.
     @style{wxSTAY_ON_TOP}
-        Makes the message box stay on top of all other windows (currently
-        implemented only under MSW).
+        Makes the message box stay on top of all other windows and not only
+        just its parent (currently implemented only under MSW and GTK).
     @endStyleTable
 
     @library{wxcore}

src/gtk/msgdlg.cpp

@@ -190,6 +190,11 @@ void wxMessageDialog::GTKCreateMsgDialog()
 
     GtkDialog * const dlg = GTK_DIALOG(m_widget);
 
+    if ( m_dialogStyle & wxSTAY_ON_TOP )
+    {
+        gtk_window_set_keep_above(GTK_WINDOW(m_widget), TRUE);
+    }
+
     // we need to add buttons manually if we use custom labels or always for
     // Yes/No/Cancel dialog as GTK+ doesn't support it natively and when using
     // Hildon we add all the buttons manually as it doesn't support too many of