http://w3c.github.io/manifest/#dfn-display-modes-values probably deserves a separate handler (has different set of values).

Question: are you sure we need dedicated manifest handler classes here? does it really give any benefit? the 'view modes' ('display values') values are used only once at application start, maybe it is enough just to have 
"Application::GetWindowShowStateXPK" and "Application::GetWindowShowStateWGT" or so?