Time Travel: Make slider into timeline

[fbarl]

In order to display different historic metadata in the Time Travel component (which would give better clues as to where we want to navigate), current slider will be made into a timeline which would be holding the metadata. The idea is to make that timeline both zoomable and pannable.

[fbarl]

@bia is currently working on the sketches for it - the very initial ones start from here.

[rade]

zoom and pan are important features in their own right, and quite independent of representing historic meta data on the timeline. So let's tackle that first. I've raised #2686 for that.

This issue is therefore only about representing events on the timeline.

[fbarl]

Some comments @foot made in #2651 (and still need to be considered):

• I got stressed by the "busy" cursor that appeared as soon as I started dragging. Maybe show that onMouseUp when stuff starts happening.
• Not a biggie but I'd prefer a "Go!" button next to the textfield, if I'm using it I'm probably making a couple of changes and it feels a bit hectic things changing around as you're trying to enter a value.
  timestamp is truncated a tiny bit for me :).
• Not sure about "live", but can't think of anything else right now.
• Show the date next to the mouse pointer when dragging? Bit of eye flicking there.

[fbarl]

Addressed by #2703.