The adder toolbar uses Shadow DOM or `!important` to prevent CSS rules
in the host page from directly affecting the styling of the toolbar.

However even inside Shadow DOM elements can still inherit CSS property
values from parent elements [1].

This commit fixes that problem for current browsers (except Edge [2]) by
using `all: initial` on the Adder's root element.

[1] See https://github.com/w3c/webcomponents/issues/314
[2] http://caniuse.com/#feat=css-all

Fixes #382