Fix CalendarStyle's layouting and grid cell calculation.

 - Wrap all panel items within a container, so that the frame width
   checks happen in only one place, and the other items don't need to
   care about it.
 - Move the logic involving grid line width into CalendarUtils, so
   that the style code can be for styling and not maths.

Task-number: QTBUG-38847
Change-Id: I99a1f131b9cd608f3f7a7627d51a75780a6cbac2
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>