clapper-gtk: Add video placeholder widget

A simple placeholder to be shown when "ClapperGtkVideo" is used with a video
sink that cannot be embedded in it (with a text explaining that).

src/lib/clapper-gtk/clapper-gtk-video-placeholder-private.h

@@ -0,0 +1,38 @@
+/* Clapper GTK Integration Library
+ * Copyright (C) 2024 Rafał Dzięgiel <rafostar.github@gmail.com>
+ *
+ * This library 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 2.1 of the License, or (at your option) any later version.
+ *
+ * This library 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 Lesser General Public
+ * License along with this library; if not, write to the
+ * Free Software Foundation, Inc., 51 Franklin St, Fifth Floor,
+ * Boston, MA 02110-1301, USA.
+ */
+
+#pragma once
+
+#include <glib.h>
+#include <glib-object.h>
+#include <gtk/gtk.h>
+
+#include "clapper-gtk-container.h"
+
+G_BEGIN_DECLS
+
+#define CLAPPER_GTK_TYPE_VIDEO_PLACEHOLDER (clapper_gtk_video_placeholder_get_type())
+#define CLAPPER_GTK_VIDEO_PLACEHOLDER_CAST(obj) ((ClapperGtkVideoPlaceholder *)(obj))
+
+G_DECLARE_FINAL_TYPE (ClapperGtkVideoPlaceholder, clapper_gtk_video_placeholder, CLAPPER_GTK, VIDEO_PLACEHOLDER, ClapperGtkContainer)
+
+G_GNUC_INTERNAL
+GtkWidget * clapper_gtk_video_placeholder_new (void);
+
+G_END_DECLS