time control: Implement a time-range input/output mode

[ghost]

Add a toggle in the Time Control view to switch between the usual mode with 1 timestamp per text field:

[start] [end] [span]

to a "range" mode:

[range] [span]

The Range field should take and display time ranges directly in the format:
[<timestamp1>, <timestamp2>]
(ignoring any whitespace).

And again, <timestamp1> and <timestamp2> could be in any of the supported timestamp formats.