TimeValidator: Accept input that has single-digit hours without a leading zero

[flauschzelle]

This would be a nice convenience feature to avoid users getting annoyed by (for example) "5:30" being rejected when "05:30" would be valid.

Would need

• adjustments to the regexes defined in the TimeFormat Enum (e.g. r'([01]?[0-9]|2[0-3]):[0-5][0-9]' instead of r'([01][0-9]|2[0-3]):[0-5][0-9]')
• and/or adding a leading zero in some cases before the end of the validation, or using something else instead of time.fromisoformat (because that requires double-digit hours).

[binaryDiv]

This makes a lot of sense. Since we already do regex parsing anyway, we can probably just get the hours/minutes/seconds from the regex groups and then create the time object with time(hours, minutes, seconds).

I'm not sure about this yet, but it might also make sense to add a strict option for strict ISO 8601 parsing (i.e. the current behaviour).