Locale-based calendar week format

[dreamscached]

At the moment the calendar is hardcoded to use sunday-saturday week format, which is not used worldwide. I suggest to use calendar module which in turn uses user's locale to determine first day of week.

After some testing it seems that it's enough just to set DAY_NAMES appropriately and update a constant in _setupDayButtons for it to seemingly work absolutely the same with the different week format.

This PR only adds monday-sunday week format (e.g. used in all of the Europe, Russia, and many other countries); in case firstweekday returns neither 0 nor 6 the calendar remains sunday-saturday.