Split into shouldReceiveLightDismiss and lightDismiss

Allows developers to configure policy from what happens during a light
dismiss. Also provided some built in policies (Light and Sticky).
Connect hardware back button to light dismiss
Connect resize to light dismiss service
Group light dismiss triggers together
Connect window blur to light dismiss service
Use a different notation for overriding mixin defaults
LightDismissableElement mixin provides defaults for all of ILightDismissable

Now it's easier to implement a new overlay. However, you have to override
ILightDismissable methods in WinJS.Class.mix instead of
WinJS.Class.define. If you put it in WinJS.Class.define then they'll be
overriden during WinJS.Class.mix.
wip type to search