Add XMonad.Layout.CenteredIfSingle

[elkowar]

Description

XMonad.Layout.CenteredIfSingle centers windows on screen as long as there is only a single window. As soon as there are more than one windows, the modified layout is used as normal.

Small demonstration:

https://user-images.githubusercontent.com/5300871/139041003-a2ceb6ae-07d1-405d-9056-4001d14e8538.mp4

Checklist

• [x] I've read CONTRIBUTING.md

• [x] I've considered how to best test these changes (property, unit, manually, ...) and concluded: Manual testing is sufficient, I've been using this for months

• [x] I updated the CHANGES.md file

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

[TheMC47]

@elkowar could you squash the commits? Do that and I'll merge

[elkowar]

Done @TheMC47