[keys] add accesskey shortcuts for incrementing/decrementing key count

[danmana]

Use "ALT + [" shortcut to decrement keys
Use "ALT + ]" shortcut to increment keys

[dingram]

Looks good, except the shortcut varies depending on browser and operating system: https://developer.mozilla.org/en-US/docs/Web/HTML/Global_attributes/accesskey

[nexushoratio]

The traditional IITC way has been to use:

title="[x]"
or
title="Action [x]"

That avoids the whole keystroke combination thing.

But I have no idea what to suggest for an alternative here as:

title="Add []]"

Does look kind of wonky.

In other projects, I've personally used angle brackets for keystrokes. Maybe that would be a good option for square brackets?

title="Add <]>"

Also, I think the functions of the keys should be reversed: [ - decrement and ] - increment.

Again, comparing to angle brackets, I think it's traditional for < to decrement and > to increment. Plus on QWERTY keyboards, [ is to the left of ].