New module: X.L.ConditionalLayout

[slotThe]

Supersedes #582

What I haven't done, yet we probably should (not necessarily in this PR):

• Rewrite IfMax, PreScreen, and PerWorkspace with this, which would close Display bug with IfMax, PerScreen, PerWorkspace with decorated layouts  #75
• Provide some QOL functions so that it's easier to write predicates. E.g., I reckon lots of people will want to use a variant of gets $ find ((t ==) . W.tag) . W.workspaces . windowset.
• I'm not sure if this is enough docs still

TODOS:

• Find out what's up with New module: X.L.ConditionalLayout #825 (comment)
• Docs and changelog entries for all the deprecations
• Docs for the exported helper functions

New module: X.L.ConditionalLayout

Provide conditional variants of 'ModifiedLayout' and 'Choose', so that modifications (specific layouts) are only applied when a particular condition is met.

Co-authored-by: Tomas Janousek [email protected]
Co-authored-by: Ivan Malison [email protected]

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Description

Include a description for your changes, including the motivation
behind them.

Checklist

• I've read CONTRIBUTING.md

• I've considered how to best test these changes (property, unit,
  manually, ...) and concluded: cursory testing on my computer yielded the desired results, though someone else should look at this.

• I updated the CHANGES.md file

[slotThe]

There's a but that I can't figure out right now: Assuming something like

myLayout = onWorkspace "4" (tall ||| Full) $ Full ||| tall ||| Mirror tall
 where
  tall = Tall 1 (3/100) (1/2)

one needs to issue five M-SPC calls to cycle through all layouts, regardless which branch one is on right now (some of those M-SPC invocations will just not do anything—at least not visibly).

Haven't tried debugging this yet, as its getting late, but perhaps someone else immediately knows what's up.

[geekosaur]

Is using Choose internally wise? That's going to add a couple extra mod-space-s and I'm wondering how that interacts with the explicit condition (wouldn't it effectively invert it?).

[geekosaur]

Also it may be better to leave PerScreen alone until we decide what's happening with #813.