Use dedicated function to change wxPGProperty flags

Using wxPGProperty::ChangeFlag() function makes the code simpler and more readable.

src/propgrid/advprops.cpp

@@ -1593,11 +1593,7 @@ bool wxSystemColourProperty::DoSetAttribute( const wxString& name, wxVariant& va
     }
     else if ( name == wxPG_COLOUR_HAS_ALPHA )
     {
-        if ( value.GetBool() )
-            m_flags |= wxPG_PROP_COLOUR_HAS_ALPHA;
-        else
-            m_flags &= ~(wxPG_PROP_COLOUR_HAS_ALPHA);
-
+        ChangeFlag(wxPG_PROP_COLOUR_HAS_ALPHA, value.GetBool());
         return true;
     }
     return wxEnumProperty::DoSetAttribute(name, value);