abstract utf8 validation fallback

abstract utf8 validation fallback

Language class had a code snippet to verify whether a text is valid
UTF-8 though that could not be used from another place. The snippet use
mb_check_encoding() and fallback to some regex whenever mbstring is not
available.

* introduce StringUtils::isUtf8() which is mostly code moved out of the
  language class.
* Enhance regex readability by using an expanded regex (//x)
* Made the regex to recognize longer sequences
* Add some unit tests to the mbstring and the PHP native implementation
* An optional second parameter can be passed to isUtf8() to force the
  use of our PHP implementation. This is used for unit testing.

Change-Id: I4cf4dfe2eb02f046db1726f4654ba649e01419f2