Dynamically update live objects

[Ambrevar]

We have define-configuration to help customizing the defaults, but this does not update the live objects when the user calls, for instance, load-init-file.

Typical example: the user changes some CSS and they want to see the results right-away (without restarting, without creating a new buffer).

One option would be to create a dedicated macro, like apply-configuration, that would work just like define-configuration, but apply the slot values to the existing buffers, windows, and the *browser*.

Thoughts?

[aadcg]

Makes sense to me @Ambrevar!

[Ambrevar]

@jmercouris As discussed, this is one of the most asked-for features. Should we work something out for 2.0?

[aadcg]

As mentioned in #1490, update-instance-for-redefined-class is a way to achieve it.