We're flagging new added rule by adding a `userAdded` property on the StyleRule
form when the rule is added from the UI.
We then refactor `_onAddRule` so it only calls the server and stores the actorID
of the new rule.
Then, in `_createEditors`, if a rule has `userAdded`, and if we are waiting for
a user added rule, we focus the rule's selector.
We take this opportunity to emit a dedicated `new-rule-added` event instead of
relying on the more generic `ruleview-changed` that we previously had.
A few test are updated in consequence.
Differential Revision: https://phabricator.services.mozilla.com/D186463
777cf52f7c