Fixed E_NOTICE in listing worker queue and new utilities class added (#5521)

Fixed E_NOTICE in listing worker queue and new utilities class added (#5521)

* Fixes for E_NOTICE in workqueue:
- introduced class `Friendica\Util\Arrays` which will hold static methods for
  handling arrays that cannot be done with PHP's functions, like implode() on
  multi-dimensional arrays
- rewrote old-school for() loop to foreach()

* Added intial unit test with some tests on empty delimiters and/or sinle and
multi-dim array.

* Added test for for 3-dimensional arrays, thanks to  nupplaphil's feedback.