feature: enhance calendar widget with time picker

[seflue]

When creating a date using the calendar widget, a time can now set directly from the widget.

• t jumps to the time picker
• h/l switches between hour, "big" minute and "small" minute selection
• j/k decreases/increases the time

The step size for "big" and "small" minutes can be configured. Default is 15 and 5 minutes. When adjusting hours, the minutes are rounded or not (this behavior can be disabled in the config, if too confusing - I find it quite useful).

See also #607.