kcms/users: Fallback to show username in title when real name isn't set

There are 4 places in QML code in this patch which use these new shorthand
properties: 3 old expressions reduced to a single model role access, and
one new expression that is basically the bug which I originally intended
to fix with one-liner, but decided to do this little refactoring instead.

(cherry picked from commit 3cc6ac73)