Categorize standard shortcuts

We have a huge amount of standard shortcuts. By adding the ability to divide them into categories, they can be presented to the user in a way that enables them to easier find the shortcut that they want. Internally some shortcuts were already grouped but that seems to have stopped for the later additions. (The related standard actions from KConfigWidgets also seem to have this not exposed internal grouping.) See plasma/plasma-desktop!26 (merged)