add alt attribute for a11y and fix double-tap event

[notwaldorf]

• adds alt attribute for a11y to fix https://github.com/PolymerElements/paper-icon-button/issues/4
• removes the duplicate button behaviour and moves the host attributes into the button to fix https://github.com/PolymerElements/paper-icon-button/issues/19 and https://github.com/PolymerElements/paper-icon-button/issues/20

[cdata]

Since you've already done such a thorough job applying alt, I must do the evil thing and ask: is alt is the best name to use for this attribute? In other words, is it better to use alt than, say, title or label or something like that?

[notwaldorf]

I picked up alt because that's what Alice suggested in https://github.com/PolymerElements/paper-icon-button/issues/4. title is bad because it's used for tooltips (and tooltip != alt). Imgs have a native alt and it already has a fairly defined meaning (alternate text to be read), so it kind of make sense?

/cc @alice for input

[alice]

As per #4, alt is also what is used in input type=button for this purpose. As @notwaldorf points out, title has a different meaning and behaves differently. I think alt is correct.

[alice]

More info on alt for input: http://www.w3.org/TR/2012/WD-html5-20121025/states-of-the-type-attribute.html#attr-input-alt

[notwaldorf]

:wave: @cdata: also added the host attributes/behaviour change to fix https://github.com/PolymerElements/paper-icon-button/issues/19 like we talked about

[notwaldorf]

@cdata Fixed all the things. :eyeglasses: :8ball: :question:

[cdata]

LGTM :+1: