theme-manager: add SpThemeManager and use it for custom css

We want to set some custom backgrounds for the visualizers, and
those need to track with the current theme. SpThemeManager will
watch the systems theme changes (including dark theme) and
update the loaded CSS resources as necessary.

lib/sp-theme-manager.h

@@ -0,0 +1,40 @@
+/* sp-theme-manager.h
+ *
+ * Copyright (C) 2016 Christian Hergert <chergert@redhat.com>
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program 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 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/>.
+ */
+
+#ifndef SP_THEME_MANAGER_H
+#define SP_THEME_MANAGER_H
+
+#include <gtk/gtk.h>
+
+G_BEGIN_DECLS
+
+#define SP_TYPE_THEME_MANAGER (sp_theme_manager_get_type())
+
+G_DECLARE_FINAL_TYPE (SpThemeManager, sp_theme_manager, SP, THEME_MANAGER, GObject)
+
+SpThemeManager *sp_theme_manager_get_default       (void);
+void            sp_theme_manager_unregister        (SpThemeManager *self,
+                                                    guint           registration_id);
+guint           sp_theme_manager_register_resource (SpThemeManager *self,
+                                                    const gchar    *theme_name,
+                                                    const gchar    *variant,
+                                                    const gchar    *resource);
+
+G_END_DECLS
+
+#endif /* SP_THEME_MANAGER_H */