Normalize modifier key names

[patrickberkeley]

Currently modifier key names are:

altKey, ctrlKey, metaKey, shiftKey

But if those keys are used as execution keys, their names are:

Alt, Control, Meta, Shift

This ticket is to normalize the modifier key names so they are the same as their execution counterpart. Example result:

  get(this, 'keyManager').addMacro({
      callback: bind(this, function() {
        this.send('toggleModal');
      }),
      executionKey: 'A',
      modifierKeys: ['Alt', 'Control', 'Meta', 'Shift']
      keyEvent: 'keydown',
    });