EWMH: support _NET_WM_STATE_{ABOVE,BELOW}

[TheMC47]

Description

Add support for NET_WM_STATE{ABOVE,BELOW}. This includes both a manage hook to handle states that are put at the start and an event handler for ClientMessageEvent.

I am not sure where to exactly hook into the hooks (not particularly proud with how I added the handle event hook), so ideas are welcome. Maybe I could do a small refactoring for the module first? Not sure

Checklist

• I've read CONTRIBUTING.md

• I've considered how to best test these changes (property, unit,
  manually, ...) and concluded: Wrote a small python script that would switch between the states (using add, remove, and toggle) and observed the _NET_WM_STATE with xprop -spy and how the window behaved

Add ABOVE - should go above other windows
Remove ABOVE - should not change
Add ABOVE again - should stay above other windows
Add BELOW - should go below other windows
Remove BELOW - should not change
Toggle ABOVE - should add ABOVE
Toggle ABOVE - should remove ABOVE
Toggle BELOW - should add BELOW
Toggle BELOW - should remove BELOW
Add ABOVE again - should go above other windows
Toggle BELOW - should add BELOW and remove ABOVE
Toggle ABOVE - should add ABOVE and remove BELOW

• I updated the CHANGES.md file

[geekosaur]

…once the typo is fixed 😀

[liskin]

How does this interact with xmonad setting the window order in X.O.windows?

[liskin]

How does this interact with xmonad setting the window order in X.O.windows?

Seems to me this only works meaningfully for unmanaged windows... If so, that seems like a rather big caveat that's worth spelling out somewhere.

[TheMC47]

It works for managed windows too. The state is not forced, so as soon as the user interacts with the window (changes focus, for example) the stacking order changes. As I understood the spec, that's expected