The use of a tri-state checkbox for the three possible states (12hr, 24hr, use locale
default) is not ideal because it violates the convention regarding what tri-state
checkboxes are used for: nested lists where some sub-items can be unselected.
This patch replaces it with a combobox that clearly indicates all three states.
While we're at it, we add a button to open the Formats KCM in case people want to change their region.