[FEATURE] Add possibility to flatten array

A commonly used task in TYPO3 is to convert and work with
several types of arrays. This change allows to flatten a
nested multidimensional array to a flat key-value array.

array('first.' => array('second' => 1))
will become
array('first.second' => 1)

Change-Id: Ia41c53b87ac984bb6b29741ce461e4af70b6215f
Resolves: #39951
Releases: 6.0
Reviewed-on: http://review.typo3.org/13910
Reviewed-by: Susanne Moog
Tested-by: Susanne Moog