clapper-gtk: video: Split code into subclass

Create a base class called "ClapperGtkAv" and subclass it into "ClapperGtkVideo".
This allows to have player and session inhibit logic in base class and share it
with other subclassed widgets.

src/lib/clapper-gtk/clapper-gtk-av.h

@@ -0,0 +1,60 @@
+/* Clapper GTK Integration Library
+ * Copyright (C) 2025 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, see
+ * <https://www.gnu.org/licenses/>.
+ */
+
+#pragma once
+
+#if !defined(__CLAPPER_GTK_INSIDE__) && !defined(CLAPPER_GTK_COMPILATION)
+#error "Only <clapper-gtk/clapper-gtk.h> can be included directly."
+#endif
+
+#include <glib.h>
+#include <glib-object.h>
+#include <gtk/gtk.h>
+#include <clapper/clapper.h>
+
+#include <clapper-gtk/clapper-gtk-visibility.h>
+
+G_BEGIN_DECLS
+
+#define CLAPPER_GTK_TYPE_AV (clapper_gtk_av_get_type())
+#define CLAPPER_GTK_AV_CAST(obj) ((ClapperGtkAv *)(obj))
+
+CLAPPER_GTK_API
+G_DECLARE_DERIVABLE_TYPE (ClapperGtkAv, clapper_gtk_av, CLAPPER_GTK, AV, GtkWidget)
+
+struct _ClapperGtkAvClass
+{
+  GtkWidgetClass parent_class;
+
+  /*< private >*/
+  gpointer padding[4];
+};
+
+CLAPPER_GTK_API
+ClapperPlayer * clapper_gtk_av_get_player (ClapperGtkAv *av);
+
+CLAPPER_GTK_API
+void clapper_gtk_av_set_auto_inhibit (ClapperGtkAv *av, gboolean inhibit);
+
+CLAPPER_GTK_API
+gboolean clapper_gtk_av_get_auto_inhibit (ClapperGtkAv *av);
+
+CLAPPER_GTK_API
+gboolean clapper_gtk_av_get_inhibited (ClapperGtkAv *av);
+
+G_END_DECLS