gatherTranslatableStrings() refactored into TranslationFile class

- Refactored lib\gatherTranslatableStrings() into a class
- Speed up on many places (not that it would be necessary here:-)
- ``__(…)`` now supports multiline, concatenated, commented and escaped arguments. So the following:
```php
    $msg = __ /* Space is for rent */

        # This is crap
        (  # useless comment
    /* comment*/ 'The ' .
            "quick \"brown\" "   .'fox \'jumps\' '

    .                "over…"  # End
        );
```
would now be found now and results in ``The quick "brown" fox 'jumps' over…``