Allow visibility of Inherited Methods to toggle

[mrdevin]

Fixes #163

when building polymer elements that extend Polymer.Element it can be hard to find your own methods in the long list of inherited methods. This solution has all inherited methods hidden by default and adds an iron-doc-hide-bar to toggle inherited methods.

It would be great if we could have something like this. Any improvements or suggestions.

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[mrdevin]

I signed it!

[googlebot]

CLAs look good, thanks!

[aomarks]

Sorry for the extremely delayed review.

I tried out this PR, and I found it confusing that when a method was both inherited and protected, you need to click both hide bars. I couldn't immediately think of a way to make that more intuitive.

I am wondering if we need a larger reorganization, where each inherited class's methods and properties is shown in its own section, and is collapsed by default.

[mrdevin]

no stress.

I think your idea to reorganize makes more sense then what i proposed. IF i get time I might make an attempt to reorganize myself but then on another branch.

thanks!