Ensure mu is accessible.

[ntoll]

It is an essential requirement that no-one is excluded from using the micro:bit. This includes those of us who require capabilities built into the editor so it is easier to use. Happily Qt has a comprehensive guide to making this happen. You can read it here:

http://doc.qt.io/qt-5/accessible.html

I believe accessibility should be a core part of our development process and that all the features of mu should be accessible to all users. We should make it a requirement for code-review that there's a step to check accessibility such that our code conforms with the suggestions in the document linked to above.