Introduced overlay-class and container-class attributes.

[lukemelia]

This commit reworks how css classes are provided to the modal-dialog component
and then handed down to the overlay div and the position-container component.
Previously, we attempted to use one property for both template-provided css
classes as well as subclass-provided css classes. That is problematic for the
same reason that Ember separates classNames and classNameBindings. After this
commit, if you want to specify additional class name(s) when you are using
modal-dialog in a hbs template, use overlay-class and/or
container-class, and if you want to specify additional class names(s) when
you are subclasses the modal-dialog component, use overlayClassNames
and/or containerClassNames. The dummy app has been updated with examples.

[sandstrom]

Out of curiosity, is there a reason these were named container-class instead of containerClass?
(the format I've mostly seen used for component properties)

[lukemelia]

@sandstrom I suspect we are going to move toward html naming style for component attributes as we start using angle bracket components. I was experimenting around that idea.

[sandstrom]

Ah, okay.