Really add wxGA_PROGRESS support to wxGauge in wxOSX

Finish the work started in 11a5b83e2c by moving
more wxAppProgressIndicator-related parts of wxMSW wxGauge implementation into
the base class and reusing them from the wxOSX version.

Also remove MSW-specific test for wxUSE_TASKBARBUTTON from the widgets sample
which prevented this style from being taken into account at all under Mac.

See #16638.

samples/widgets/gauge.cpp

@@ -293,10 +293,8 @@ void GaugeWidgetsPage::CreateGauge()
     if ( m_chkSmooth->GetValue() )
         flags |= wxGA_SMOOTH;
 
-#if wxUSE_TASKBARBUTTON
     if ( m_chkProgress->GetValue() )
         flags |= wxGA_PROGRESS;
-#endif
 
     int val = 0;
     if ( m_gauge )