Animate the button box. Add some padding to the class structure for ABI protectiness.
Not everyone wants to preview the previous/next tile when they hover on the tile edges. Make this behaviour optional.
This also fixes dangling callback problems. See: http://bugzilla.clutter-project.org/show_bug.cgi?id=2689
g_set_error() already checks whether **error == NULL, the extra check is just pointless branching, which is somewhat ironic when someone has gone out of their way to provide branch prediction hints.