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

fixed wrong language name definition

adding and reworking the conversion of:
- ICU to PHP
- PHP to ICU
- ICU to JUI
- PHP to JUI

This fix ensures the format configured in the formatter class is used in the message instead
of the default ICU format.

fixes #4960

ensure some features that were recently added to formatter keep working after refactoring

exclude parts that have different default values

fixes #2519: MessageSource removed translation messages when event handler
was bound to `missingTranslation`-event

- DbMessageSource now performs UNION ALL.
- Fixed message merging for php and po.
- Updated tests.

fixed #1021

- refactored replaceNamedArguments.
- ensure gridview message works on all systems.

fixes #1072

Added intl check to tests, better error reporting, credited Aura.Intl for the most complex test pattern