EDateEdit: Force 24 hour format for locales without am/pm equivalents

Some locales do not provide equivalents for am/pm (the "%p" of strftime),
thus when the user has set 12 hour format in Preferences, then the am
and pm times are the same, which confuses the widget, especially
in the appointment editor, making it hard (like impossible) to enter
pm times by selection in the time combo. An example locale is de_DE.