Handle beat-tracking for starts/stops

[oubiwann]

Only one start was allowed; let's allow for any number of starts/stops.

Tasks:

• [x] Come up with good name and update the data structures
• [x] Refactor transport into an actual abstraction (and with its own constructor)
• [x] Transport could be used anywhere, so put it in um.transport
• [x] Update all transport-related logic to use new abstraction
• [x] Put ms <-> s time conversions into a function
• [x] Provide some basic/common time functions for better reuse
• [x] Fix padding of H:M:S in formatted times to HH:MM:SS

Part of:

• 42