Tweak README-KeyTab and README.keyboard a bit

Summary:
Rename README.KeyTab to README-KeyTab to prevent Konsole from trying to
load it as a keyboard translator file, and fix references to that file
name in other files.

Flesh out README-KeyTab and README.keyboard and reformat them.

Also copy the the original README-KeyTab to README-KeyTab-original,
just in case there's some concept that I misinterpretted/misunderstood.

Reviewers: #konsole, hindenburg

Reviewed By: #konsole, hindenburg

Subscribers: #konsole

Tags: #konsole

Differential Revision: https://phabricator.kde.org/D12032