extracted date format converting to a helper class

adding and reworking the conversion of:
- ICU to PHP
- PHP to ICU
- ICU to JUI
- PHP to JUI
Added tests for empty values

use "just now" for empty time difference in Formatter.

fixed wrong language name definition
Refactored DateValidator to support ICU format
Merge pull request #5027 from yiisoft/date-format-convert

[WIP] extracted date format converting to a helper class