provide a way to pause/resume sets of systems

[mreinstein]

There are some cases where a number of systems should be turned on and off as a group. For example, when an in-game menu is open, and we want to pause the physics, sprite animations, player input, and timers, but we don't want to pause the menu system or the renderer.

Currently I have logic at the beginning of every system I want to pause that essentially looks like:

const onFixedUpdate = function () {
      if (Global.paused)
          return

      ...
}

But this is kind of ugly. It might be nice to offer a way to register systems as part of a set, and pause/resume that set.