shell: add a autocleanup function to CcPanel

CcPanel uses the old boilerplate code from GLib,
which does not set an autocleanup function.

The lack of a cleanup function implies that panels
cannot use G_DECLARE_{FINAL,DERIVABLE}_TYPE, making
the code stick to the old boilerplate.

This patch adds a cleanup function to CcPanel. It doesn't
move CcPanel to G_DECLARE_DERIVABLE_TYPE() because it'd
break the CcPanel's subclasses.

https://bugzilla.gnome.org/show_bug.cgi?id=769063

shell/cc-panel.h

@@ -50,6 +50,8 @@ typedef struct CcPanelPrivate CcPanelPrivate;
 typedef struct _CcPanel       CcPanel;
 typedef struct _CcPanelClass  CcPanelClass;
 
+G_DEFINE_AUTOPTR_CLEANUP_FUNC (CcPanel, g_object_unref)
+
 /* cc-shell.h requires CcPanel, so make sure it is defined first */
 #include "cc-shell.h"