Fixes #2527: Source language for `app` message category was always `en` no matter which application `sourceLanguage` was used

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

Fixes #2157: The '*' category pattern will match all categories that do not match any other patterns listed in `I18N::translations`

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

- i18n now falls back to `en` from `en-US` if message translation isn't found
- View now falls back to `en` from `en-US` if file not found
- Default `sourceLanguage` and `language` are now `en`

Appended newlines to every PHP document that did not have a newline as the last character in the file.

I18N WIP.