egg: bring multi-paned ported to GTK 4

This is out of libpanel, which already has it but with the dock stuff
removed (more like DzlMultiPaned). We probably still need some work to
handle natural allocation from children.

src/libsysprof-ui/egg-handle-private.h

@@ -0,0 +1,35 @@
+/* egg-handle.h
+ *
+ * Copyright 2021 Christian Hergert <chergert@redhat.com>
+ *
+ * This file is free software; you can redistribute it and/or modify it under
+ * the terms of the GNU Lesser General Public License as published by the Free
+ * Software Foundation; either version 3 of the License, or (at your option)
+ * any later version.
+ *
+ * This file is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public
+ * License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ * SPDX-License-Identifier: LGPL-3.0-or-later
+ */
+
+#pragma once
+
+#include <gtk/gtk.h>
+
+G_BEGIN_DECLS
+
+#define EGG_TYPE_HANDLE (egg_handle_get_type())
+
+G_DECLARE_FINAL_TYPE (EggHandle, egg_handle, EGG, HANDLE, GtkWidget)
+
+GtkWidget *egg_handle_new          (GtkPositionType  position);
+void       egg_handle_set_position (EggHandle       *self,
+                                    GtkPositionType  position);
+
+G_END_DECLS