Qt supports this string representation for Qt::KeypadModifier since 5.1
but this code didn't support that modifier.
This is the easy part of the fix for the 8-years-old bug, and it makes
it possible to use the numeric keypad for application shortcuts.
The bigger issue is handling of keypad modifiers in global shortcuts
(patches to come for kwindowsystem and kglobalaccel).
CCBUG: 183458