[trunk] Shortcuts cannot be cleared

Setup: trunk rev. 4220 both under Win XP (Qt lib 4.7.0) and Ubuntu (Qt lib 4.6.2)

Steps:

1) Start MuseScore
2) Menu "Edit | Preferences", tab "Shortcuts"
3) Select any shortcut and press [Clear]: the shortcut is cleared
4) Press [OK] to close the dlg box
5) Close MuseScore
6) Start MuseScore again
7) Menu "Edit | Preferences", tab "Shortcuts" again

Result: the shortcut cleared in step 3) is still defined.

This applies to both built-in shortcuts and to user-created shortcuts.