add component-laid-out lifecycle callback

[Andersmholmgren]

When working with layout managers like polymer-layout, polymer-grid-layout etc it is often important to know when the component has finished being layout out. For example, if the component contains a canvas element that needs to be resized based on the containing element this needs to happen when the containing element has it's size set on completion of the layout.

polymer-grid-layout has it's own event that it fires as polymer-layout may soon but this requires use of the signal mechanism at this stage. These, whilst useful, seem somewhat adhoc, given this is IMO a key lifecycle event for a component

[sjmiles]

As of now this is something to solve at a component or component-set level. There is no clear mandate today for what this would mean more generally.

This is not an inherently bad idea, but I think we need to let it rise again later.

[Andersmholmgren]

I'm updating my dart polymer app to the latest polymer, core-elements and paper-elements. I was previously using using polymer-elements and in that polymer-layout had a on-polymer-layout event that I was using to help size a canvas element to take the entire size of it's container (see http://stackoverflow.com/questions/21816467/autosizing-canvas-inside-a-polymer-grid-layout).

I can't find an equivalent int core-elements so I'm back to square one. How do I handle this now?

See also http://stackoverflow.com/questions/22111447/there-doesnt-seem-to-be-a-polymer-layout-event