New helper method PET_Utility::InvokePET

InvokePET allows to use a PET without the need of enabling post
processing.
All plugins that need PETs must use InvokePET to ensure they can work
even with post processing disabled.
New ExistingVirtual helping method

ExistingVirtual allows to create an Entity based the passed values
without going to the database. This prevents additional round trips to
the database when possible.
Fix for EscapeString in old PHP versions
Updated input.pet.php

- Types:
-- literal -> the value is wrapped in span
-- html -> the value is wrapped in div
-- textarea -> small fix
-- select -> using PET_Utility::InvokePET
HTML5 fix clear="both" is obsolete
Fixed typos
All is_null replaced to === null

!is_null replaced to !== null
is_null replaced to === null