Adding ability to show menu button content and keep focus on it.

[vashchenkov]

It's needed to make dropdown menu with filter.

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[vashchenkov]

This commit was made by me using another mail, which was added as extra email at my google account after I have assigned CLA with my google account.

[valdrinkoshi]

With the latest paper-menu-button you can leverage the property noOverlap which will render the dropdown below/above the trigger element.