Fix layout of property signal table with const properties.

Ignore-this: 727342cf44e715ab9edf1fb6d87fc046

darcs-hash:20140607113119-4d2ae-903eef7dab657da2c320b7298e75ab1ffb33987d
Add a const property to test suite.

Ignore-this: fe6d4be233e4f868e4889bf2057c4692

darcs-hash:20140607113947-4d2ae-56d4040d582fea9dd4f59e132d4aea2abbe0324e