[components/calendar][applets/digital-clock]: Remove the partial fork of header in digital-clock and unify the headers

The digital clock uses a slightly different layout in the header and has a bad fork of header from calendar. This may seem not to be a problem but header and the calendar body is deeply related. Changes to calendar always break the digital clock and vice-versa. It is too annoying to port any changes in the calendar component to the header and almost always causes key navigation problem, focus loss and other issues. Controlling them in one place makes them maintianable and more pleasant to work with.