Add a displayName property on the NodeActor, which compute from Element.prefix + Element.localName.
The computation is made by a getNodeDisplayName function which can be imported wherever needed.
Edit some tests to ensure we correctly display node names.
MozReview-Commit-ID: 6z0G3ynbMoU
ESLint 2 now flags anonymous generator functions according to
generator-star-spacing. Most of the changes here are correcting that.
MozReview-Commit-ID: 9xg2Gmlaz94
The main change here is that nodeFronts that have already been displayed
in the timeline are stored in a WeakMap so they can be retrieved from it
next time they're displayed and avoid a server-side round trip which, in
turn, causes the UI to flicker.
The other change is that now, it is possible to tell the animations actor
what is the current walker actor, which allows animation player actors to
directly send the NodeActor ID as part of their forms. Which, in most cases,
completely eliminates the server round-trip, because the corresponding
NodeFronts are already known on the client, so we get them from there.
The last change done here is that AnimationTargetNode now becomes a thin
wrapper on top of the new DomNodePreview component that was extracted so
it can be reused in other places.
