QKeySequenceEdit can't register shortcuts with dead keys

Shortcuts with a Dead keys could never be registered.
Entered in QKeySequenceEdit widget, these shortcuts produced a garbage
in place of the dead key, and the resulting shortcut couldn't be
triggered.

For example, on German layout the key below Esc is a Dead Circumflex key.

The test case:

• set German layout in your System settings
• open any app with QKeySequenceEdit or KKeySequenceWidget (that one
  could be found in Plasma Shortcut Settings) widget, or create a test form with that widget
• try to register any shortcut with Dead Circumflex key (a key below
  Esc), e.g.: Alt+`Dead Circumflex`.

Expected result:
the widget should show Alt+<some_meaningful_deadkey_name> Key Sequence,
the shortcut registered should trigger corresponding action

Actual result:
the widget shows weird "Alt+ោ?" Key Sequence, the shortcut can never be
triggered