Scroll by clicking on scrollbar. Fixes #44

To fix this, I had to move the `on-tap` handler to the `<core-icon>`. Otherwise, `<paper-menu-button>` would be the event target and I wouldn't be able to detect if `<core-icon>` was clicked or something else.

Another part of the fix is to set a negative z-index for `.paper-menu-button-overlay-bg`. Currently, this background is covering the scrollbar and making it impossible to grab (you can verify this by changing Mac System Preferences to always show scroll bars).

Reviewers @sorvell and @frankiefu