It's often difficult to make contrib modules work together. When one
depends on a functionality of another, it is often necessary to expose
lots of low-level functions and hooks and have the user combine these
into a complex configuration that works. This is error-prone, and
arguably a bad UX in general.
This commit presents a simple solution to that problem inspired by
"extensible state": extensible config. It allows contrib modules to
store custom configuration values inside XConfig. This lets them create
custom hooks, ensure they hook into xmonad core only once, and possibly
other use cases I haven't thought of yet.
This requires changes to xmonad core: https://github.com/xmonad/xmonad/pull/294
A couple examples of what this gives us:
* [X.H.RescreenHook](https://github.com/xmonad/xmonad-contrib/pull/460)
can be made safe to apply multiple times, making it composable and
usable in other contrib modules like X.H.StatusBar
* `withSB` from X.H.StatusBar can also be made safe to apply multiple
times, and we can even provide an API [similar to what we had
before](https://hackage.haskell.org/package/xmonad-contrib-0.16/docs/XMonad-Hooks-DynamicLog.html#v:statusBar)
if we want (probably not, consistency with the new dynamic status bars
of https://github.com/xmonad/xmonad-contrib/pull/463 is more important)
* The [X.H.EwmhDesktops refactor](https://github.com/xmonad/xmonad-contrib/pull/399)
can possibly be made without breaking the `ewmh`/`ewmhFullscreen` API.
And we will finally be able to have composable EWMH hooks.
Related: https://github.com/xmonad/xmonad/pull/294
