Resolves: tdf#99604 ungrab modal dialogs

Change-Id: I6735913c371968745da3ad6bf61e6ae3287c19a6
(cherry picked from commit 011ce226e89ecabaf621603d692547c88061eaba)
diff --git a/vcl/unx/gtk3/gtk3gtkframe.cxx b/vcl/unx/gtk3/gtk3gtkframe.cxx
index 576946f..aace8b3 100644
--- a/vcl/unx/gtk3/gtk3gtkframe.cxx
+++ b/vcl/unx/gtk3/gtk3gtkframe.cxx
@@ -2377,6 +2377,13 @@ void GtkSalFrame::SetModal(bool bModal)
    if (!m_pWindow)
        return;
    gtk_window_set_modal(GTK_WINDOW(m_pWindow), bModal);
    if (bModal)
    {
        //gtk_window_set_modal bTrue adds a grab, so ungrab here. Quite
        //possibly we should alternatively call grab_add grab_ungrab on
        //show/hide of menus ?
        gtk_grab_remove(m_pWindow);
    }
}

gboolean GtkSalFrame::signalButton( GtkWidget*, GdkEventButton* pEvent, gpointer frame )