context: Make ClutterContext a GObject and move to separate file

This is a step in cleaning up the Clutter context management. By making
it a GObject it's easier to add e.g. properties and features that helps
with introspection.

For now, this means the context creation is changed to go via a
"constructor" (clutter_create_context()). This is so that the global
context singleton can be mantained outsid of ClutterContext, until it
can be removed.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2016>

clutter/clutter/clutter-context-private.h

@@ -0,0 +1,48 @@
+/*
+ * Copyright (C) 2006 OpenedHand
+ * Copyright (C) 2023 Red Hat
+ *
+ * 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 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 <http://www.gnu.org/licenses/>.
+ *
+ */
+
+#pragma once
+
+#include "clutter/clutter-context.h"
+
+struct _ClutterContext
+{
+  GObject parent;
+
+  ClutterBackend *backend;
+  ClutterStageManager *stage_manager;
+
+  GAsyncQueue *events_queue;
+
+  /* the event filters added via clutter_event_add_filter. these are
+   * ordered from least recently added to most recently added */
+  GList *event_filters;
+
+  CoglPangoFontMap *font_map;
+
+  GSList *current_event;
+
+  GList *repaint_funcs;
+  guint last_repaint_id;
+
+  ClutterSettings *settings;
+
+  gboolean is_initialized;
+  gboolean show_fps;
+};