[Debug] [4.2] Structure debug toolbar timeline with collapsible elements.

[sfadschm]

This is an extension of #4865.

This PR intrduces structuring of the debugbar timeline elements.
They are now sorted in a parent-children structure like this:

[
    'name'     => 'parentTimer',
    'start'    => '0',
    'duration' => '10',
    'children' => [
        [
            'name'     => 'childTimer',
            'start'    => 2,
            'duration' => 4
        ]
    ]
]

These elements are then echoed recursively and embedded in JS collapsible table rows.

For now, the Controller parent element is unfolded by default, as this will likely alway be the biggest and last part of the timeline.

One thing I am not yet fond of is the flexible indentation of the Timer name in the current state. That is, each deeper level is indented by an additional 10px, which I think might pose some issue on very narrow screens.

Demonstration

Timeline.Demonstration.mp4

Checklist:

• Securely signed commits
• Component(s) with PHPdocs
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[MGatner]

This looks great! Thanks so much - I've often wanted this "drill down" ability, especially in Controller.

[lonnieezell]

Looks great. Thanks!