datetime - Add top bar clock and calendar switches for changing the clock

As seen on this mockup:

These switches should change:

• Whether or not the clock display has the week day (e.g. Mon).
• Whether or not the clock display has the date (e.g. Jun 6).
• Whether or not the clock display has seconds (e.g. 12:30:45).
• Whether or not the clock's popup calendar should show the week's number.