Add support for Thai year format in datepicker module

[panuta]

Year number that is used in Thailand is 543 ahead of western year number.

For example, says this year is 2009 in western format, in Thailand it will be 2552, which was mathematically calculated by adding 543 to 2009.

Normal localization method is not capable of doing this. I suggest to add an option in datepicker specifying how many years to be added to normal year number, for displaying purpose only.

[Jörn Zaefferer]

So when asking the datepicker for the year, you still want to get the Western-based year back?

[panuta]

Yes, it should return western-based year back. Both set and get action should require using Gregorian year as usual. Supporting Thai year will affect only when displaying the calendar to user.

YUI Calendar does support this feature by setting YEAR_OFFSET option to specify an offset value from Gregorian year. They have an example using this option for Taiwan calendar ​http://developer.yahoo.com/yui/examples/calendar/taiwan.html

So this option does not only benefit to Thai localization, but Taiwan as well.

[Scott González]

Duplicate of #5789.