Updated to use showable and removed closable option

This update changes a couple of things:

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#### Uses `segmentio/showable` for show and hide.
This was functionality that was being used in the dialog, the overlay and in the confirmation component. Pulled this out as a mixin

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#### Updated class name
Updated to use `.Overlay` to match the selector style of other components.

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#### Updated styles
Removed everything except the structural styles. The styles will be covered in the `aphrodite-overlay`

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#### Remove closable option
Overlays are always used as sub-components of other things, they're never used on their own. So the closing option was being doubled up. You'd have lots of `.closable()` things everywhere and it was confusing.

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#### Remove click closing
This just emits a 'click' event that the parent view can listen to and do whatever it wants. Again, this is to keep the "closing" option in a single spot.

/cc @ianstormtaylor