Highlight code changes for a specific timestamp

[nununo]

Currently, the timestamps dropdown will "time travel" whenever a new one is selected.

Instead, add a "Time travel" button to trigger that functionality.

And now, whenever a new timestamp is selected, highlight the code lines related to this particular timestamp. It should probably also have a warning saying that only surviving code done on that moment is highlighted. Code which was modified or deleted later in time will not be shown as this is not a DIFF tool.