mb_string the helpers in General

String helpers in the general class are usefull, but supporting mb_string
should be an important preoccupation for us, especially for i18n. This commits
create two new static functions: `General::strlen` and `General::substr` which
will try to call `mb_strlen` and `substr` but will fallback if they are not
available.

All other string helpers (substrmin, substrmax, right, left) have been updated to
call our implementation, so they now all support mb_string.
use our new General mb_string helpers
Possible fix for the General class not in doc

I think this may solve why the General class is not in the api docs.
see: http://www.getsymphony.com/learn/api/2.4/toolkit/general/
Introducing preparePlainTextValue

This method splits in two prepareTableValue'S responsability.
By extracting how plain text value are prepared, lots of strip_tags
hacks can be removed in the core.