gtk3: use window groups so modal dialog are modal to their toplevel frame only

so e.g. launching help from a modal dialog gives a new toplevel window
which is not blocked by the modal dialog on the other window.

likesize can go from one blocked e.g. writer window to calc and type away in
there happily

Change-Id: Id9376b393514e91dfd667dfce132f1f37367084e

vcl/unx/gtk3/gtk3gtkframe.cxx

@@ -796,8 +796,10 @@ GtkSalFrame::~GtkSalFrame()
 
     InvalidateGraphics();
 
-    if( m_pParent )
+    if (m_pParent)
+    {
         m_pParent->m_aChildren.remove( this );
+    }
 
     getDisplay()->deregisterFrame( this );
 
@@ -1196,6 +1198,12 @@ void GtkSalFrame::Init( SalFrame* pParent, SalFrameStyleFlags nStyle )
         if (!(m_pParent->m_nStyle & SalFrameStyleFlags::PLUG))
             gtk_window_set_transient_for( GTK_WINDOW(m_pWindow), GTK_WINDOW(m_pParent->m_pWindow) );
         m_pParent->m_aChildren.push_back( this );
+        gtk_window_group_add_window(gtk_window_get_group(GTK_WINDOW(m_pParent->m_pWindow)), GTK_WINDOW(m_pWindow));
+    }
+    else
+    {
+        gtk_window_group_add_window(gtk_window_group_new(), GTK_WINDOW(m_pWindow));
+        g_object_unref(gtk_window_get_group(GTK_WINDOW(m_pWindow)));
     }
 
     // set window type
@@ -2343,11 +2351,17 @@ const SystemEnvData* GtkSalFrame::GetSystemData() const
 
 void GtkSalFrame::SetParent( SalFrame* pNewParent )
 {
-    if( m_pParent )
-        m_pParent->m_aChildren.remove( this );
+    if (m_pParent)
+    {
+        gtk_window_group_remove_window(gtk_window_get_group(GTK_WINDOW(m_pParent->m_pWindow)), GTK_WINDOW(m_pWindow));
+        m_pParent->m_aChildren.remove(this);
+    }
     m_pParent = static_cast<GtkSalFrame*>(pNewParent);
-    if( m_pParent )
-        m_pParent->m_aChildren.push_back( this );
+    if (m_pParent)
+    {
+        m_pParent->m_aChildren.push_back(this);
+        gtk_window_group_add_window(gtk_window_get_group(GTK_WINDOW(m_pParent->m_pWindow)), GTK_WINDOW(m_pWindow));
+    }
     if( ! isChild() )
         gtk_window_set_transient_for( GTK_WINDOW(m_pWindow),
                                       (m_pParent && ! m_pParent->isChild(true,false)) ? GTK_WINDOW(m_pParent->m_pWindow) : nullptr