Cleaned up Menu._run().

MILESTONE: all _run() methods now implement the same signature:

    core/ScoreManager.py
            def _run(self, pending_user_input=None):

    editors/Editor.py
            def _run(self, pending_user_input=None):

    iotools/Menu.py
            def _run(self, pending_user_input=None):

    iotools/Selector.py
            def _run(self, pending_user_input=None):

    iotools/UserInputGetter.py
            def _run(self, pending_user_input=None):

    managers/Manager.py
            def _run(self, pending_user_input=None):

    wizards/InstrumentCreationWizard.py
           def _run(self, pending_user_input=None):

    wizards/InstrumentSelectionWizard.py
           def _run(self, pending_user_input=None):

    wizards/PerformerCreationWizard.py
            def _run(self, pending_user_input=None):

    wizards/PitchClassTransformCreationWizard.py
           def _run(self, pending_user_input=None):

    wizards/ReservoirStartHelperCreationWizard.py
           def _run(self, pending_user_input=None):

    wizards/Wizard.py
           def _run(self, pending_user_input=None):

    wranglers/Wrangler.py
            def _run(self, pending_user_input=None):