Changes the currentState property of SC.View to viewState to avoid conflicts with other statecharts layered on top, such as objectState and enabledState.
Fixes from the Office for Prevention of Redundancy Office: the 'focus' and 'disabled' classes are set accordingly on all SC.View subclasses.  This removes yet one more display observer from SC.CollectionView.
Tidies up some internal method names to make it easier to remember their purpose.
Reworks the isEnabled addition in order to allow for the enabled state to actively cascade to child views.  While you could previously use the `isEnabledInPane` property to determine if any ancestor view had been disabled, it was lazily computed and not bindable.  With this change, `isEnabledInPane` is updated actively, which means that it can be used to update the child view's display if wanted.  For example, a view gets the 'disabled' class if isEnabled is set to false, but it can also add isEnabledInPane as a displayProperty and use it to appear disabled if any ancestor becomes disabled.

This cascading can be blocked by any child view by setting `shouldInheritEnabled` to false, which allows you to set isEnabled on the top pane, but keep a section of child views separately enabled.

Includes unit tests.  Closes #638.
Removes all redundant isEnabled display property observers and render code.  This is already part of SC.View.
Ties all the event handling for views to isEnabledInPane, which will be false if the view is disabled or any ancestor of the view is disabled.