readonly mode

[guss77]

Add support for readonly attribute on paper-toggle-button.

I'm not sure about my use of overriding IronButtonState's event handler, but that's the only way I could find to capture the tap early enough to be canceled elegantly, but more specifically I'm not sure about the syntax for calling the original event handler - though it seems to work well, at least on Polymer 2.

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[guss77]

I signed it!

[googlebot]

CLAs look good, thanks!

[cdreisbach]

Any progress on this PR? Would be nice to implement it in the master as soon as possible.