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

In JavaScript: `{"title":"Foo"} !== {"title":"Foo"}`
Underscore.js provides an isEqual function to compare two values to see if they have the same contents, which can be use instead.
Because only one function of Underscore.js is needed we just add a copy of `_.isEqual` to customize-base.js.

props westonruter.
fixes #26061.

git-svn-id: https://develop.svn.wordpress.org/trunk@26548 602fd350-edb4-49c9-b593-d223f7449a82