keysequencehelper: clarify key naming for single-key sequences

Even though everything referred to here is a key on the keyboard, a lot of people aren't used to seeing key names with otherwise human-readable text. For keys like this that don't consist of a modifier key and another key, let's clarify the situation by changing the string a bit.

Companion to plasma/plasma-desktop!2108 (closed); the logic is the same, so I will merge this or close it as a result of the decision made regarding that one.