XMonad.Hooks.ScreenCorners: Add support for edges

[sylecn]

Description

In XMonad.Hooks.ScreenCorners, previously only corner is supported, now support for top and bottom edges is added. This is similar to Hot Edge shell extension in GNOME.

I could add support for SCLeft, SCRight too, if it's desired. I thought I'd ask for a review on this first.

Checklist

• [x] I've read CONTRIBUTING.md

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

  I did manual test on this added feature.
  SCTop and SCBottom works. They also with with corner specifier when the corner and edge overlap.

• [x] I updated the CHANGES.md file

[sylecn]

Hi, I have added support for SCLeft and SCRight, updated the doc and commit message as well. Code tested manually.

[geekosaur]

Thanks!