Port over an updated WindowController.js

[ColdHeat]

There is an updated version of WindowController.js that doesn't rely on magic numbers for timing between tabs.

https://github.com/fastmail/overture/blob/master/source/application/WindowController.js

However it relies on code that's internal to this Overture library. We should try to port over this code and use just pure JS so we can update WindowController.

I don't think this is absolutely necessary but the code looks cleaner.

[ColdHeat]

This could be related to this: CTFd/core-beta#35

[ColdHeat]

An interesting possible solution to the problem: https://greenvitriol.com/posts/browser-leader