Hide status for a blank tab; right-align speed/percentage/time; general changes.

When creating a new blank tab, the status of the current tab remained.

Since the speed, percentage and time values always use the same number of decimals, make them right-aligned to stop them from shifting at all.

And just some minor stuff:
1. use `Date.now()` rather than `new Date()` & `.getTime()`;
2. use `slice` to space-pad rather than a loop;
3. `round` followed by `toFixed` was redundant, and `toFixed` is already a string;
4. add the extra parameter to `onStatusChange` & `onSecurityChange`, fwiw;
5. there's no day, so don't mod 24 the hours;
6. no need for `timeString`, it can be made from `slimTimeString`.