Minor refactor
Use a std::array instead of a map to hold 4 items; the index into the array corresponds to the value of each enum member in KJob::Unit.
Also show a warning when any method that takes a Unit arg is called on an out-of-bounds value, that "worked" with a QMap as it would create a default constructed key (Unit).
Given Jobs are used all over the place in KDE code, it seems wasteful to use two QMaps each with 4 items.
This is inspired by some code in qtbase.
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