Datepicker: rename select method to _select

Make select() a private method as it's not part of the specification.
Datepicker: fix and simplify icon trigger demo
Datepicker: use short instead of abbreviated format for table header

Revert to old behavior with two instead of three chars.
Fixes layout issue with wrong day table cell width.
Datepicker: remove unneeded comma
Datepicker: remove unneeded default setting of option dateFormat
Datepicker: fix populate alternate field demo
Datepicker: adjust date.js day name unit test to short name change
Datepicker: restore old icon trigger demo behavior