Removes the restriction that render delegate data sources can only retrieve displayProperties properties.  This restriction is not especially helpful, but worse than that, it forces us to have excess display properties, which means excess observers being set up and running although not every property that effects the display necessarily needs to be observed.  For example, SC.ButtonView has several internal observers on properties that are also display properties.  It's more efficient to use those same observers to call displayDidChange and not have the properties also be display properties.
Removes isEnabled displayProperty from SC.Control and instead makes it an effective display property of all SC.Views by calling displayDidChange in the existing isEnabled observer on all SC.Views.
Removes three extra observers from SC.ButtonView instances by using existing observers to also call displayDidChange.