Show upper & lowercase letters in KLettres

Summary:
I've started using KLettres to help teach my little boy to recognise letters and associate their sounds, and to find them on the keyboard, and I'm finding KLettres most useful - so thank you!

One point I notice that makes it a little tricky is the difference between the upper-case letter displayed, and the lower-case version on the kid-friendly keyboard I got him (in hindsight, I kinda wish I'd got an alternative model which has both upper and lowercase labels!)

I think it'd be a really useful if KLettres displayed both the upper and lowercase version of the letter, e.g. "A / a" - both to teach the association that both of those are the same letter, even though they look a bit different, and also to make it easier to match with the label on the keyboard.

What do you think?  I'm mostly a backend Perl guy at heart, so I'm sure there's a cleaner/better way to do it than what I've done, but I've tested it and it does work.

The only potential sticking point I can see is that for some writing systems, upper/lower case might not make sense for certain scripts - Arabic/Hebrew etc, as far as I'm aware, don't really have a concept of capital letters.  That said, I don't think any such unicameral scripts are currently supported - but I've made it only append the lowercase version if toLower() returned  something different, so it should be safe if any are added in future.

Reviewers: mlaurent

Subscribers: aacid, shubham, narvaez, apol, kde-edu

Tags: #kde_edu

Differential Revision: https://phabricator.kde.org/D18764