This is the first step towards the animation-inspector UI v3 (bug 1153271).
The new UI is still hidden behind a pref, and this change doesn't implement
everything that is in the current v2 UI.
This introduces a new Timeline graph to represent all currently animated nodes
below the currently selected node.
v2 used to show them as independent player widgets. With this patch, we now show
them as synchronized time blocks on a common time scale.
Each animation has a preview of the animated node in the left sidebar, and a time
block on the right, the width of which represents its duration. The animation name
is also displayed.
There's also a time graduations header and background that gives the user information
about how long do the animations last.
This change does *not* provide a way to know what's the currentTime nor a way to
set it yet.
This also makes the existing animationinspector tests that still make sense with
the new timeline-based UI run with the new UI pref on.
This adds a generic walker method that cna be used to retrieve AnimationPlayerActor's
target DOM nodes in a way that they can be highlighted/selected later in the inspector.
Making the AnimationPlayerActor return the nodeActor directly wouldn't fully work, as
the walker would have missed the missing parent nodeFronts updates.
On the UI, this adds a new component in each PlayerWidget that displays a preview of the
target DOM nodes, highlight them on hover, and select them on click.
