Consistently handle orientation setting throughout ViewBase and MixDeviceWidget

The ViewBase retains the master orientation setting, as a separate
Qt::Orientation value; it is eliminated from ViewFlags. ViewBase
subclasses use this setting. MixDeviceWidget subclasses ask their
associated view for the same setting.

ViewBase subclasses (ViewSliders and ViewDockAreaPopup) query the
appropriate application setting via their virtual orientationSetting().