Customizer: Don't trigger a change event if two unchanged object values are equal, second pass.

Make Underscore.js a dependency for `customize-base` and use `_.isEqual()` to compare the values.
(Underscore.js was already enqueued via wp-util.js for Widgets.)

props westonruter.
fixes #26061.

git-svn-id: https://develop.svn.wordpress.org/trunk@29907 602fd350-edb4-49c9-b593-d223f7449a82