Reset opacity check box value when control settings are reset in ColourPicker demo.

All settings should be reset to default values when 'Reset' operation is invoked.

samples/widgets/clrpicker.cpp

@@ -210,6 +210,7 @@ void ColourPickerWidgetsPage::Reset()
 {
     m_chkColourTextCtrl->SetValue((wxCLRP_DEFAULT_STYLE & wxCLRP_USE_TEXTCTRL) != 0);
     m_chkColourShowLabel->SetValue((wxCLRP_DEFAULT_STYLE & wxCLRP_SHOW_LABEL) != 0);
+    m_chkColourShowAlpha->SetValue((wxCLRP_DEFAULT_STYLE & wxCLRP_SHOW_ALPHA) != 0);
 }