(wip) Refactor EWMH support

[liskin]

Description

Work in progress on #396 and on https://github.com/xmonad/xmonad-contrib/pull/109#issuecomment-259766760 (a blast from the past, but we do have all the necessary infrastructure for an extensible framework now!) and on https://github.com/xmonad/xmonad-contrib/issues/396#issuecomment-722009341.

Replaces/supersedes #399, ~which will be closed once all its functionality is implemented here~ which will be closed in favor of a conservative temporary implementation based on ExtensibleConf in #626, to be merged before the release, and work on a new modular EWMH support will continue here in #625 after the release.

Related: #396 (not really, just for GitHub metadata)

Checklist

• [X] I've read CONTRIBUTING.md

• [ ] I've considered how to best test these changes (property, unit, manually, ...) and concluded:

  This will ideally be just a refactor, and we'll be more worried about user experience than code bugs anyway. That being said, I don't expect to even try running the code for a couple days. :-)

• [ ] (TODO) I updated the CHANGES.md file

• [ ] (TODO) I updated the XMonad.Doc.Extending file (if appropriate)

[liskin]

As mentioned elsewhere, during the implementation of this I realized that everything's in place for a less invasive solution, which is being worked on in #626, so this PR is now moved to the v0.18 milestone, to be finished post-release.