Add api/v3/Timetrackinvoice.php (get).
add templates/CRM/Timetrack/Page/Snippets/AccordionTable.tpl for tables in an accordion.
CRM_Timetrack_Utils::roundUpSeconds(): round() the result, since we might round to 1 minute.
CRM/Timetrack/Case/Page/CaseView.php: add list of invoices.