increment expensive function count in WCategory::__construct() (v 2.6.2)

* use Runtime::getParser() (PhpTags >= 2.6.2)
* use Runtime::incrementExpensiveFunctionCount() (PhpTags >= 2.6.2)
* add cache for minimize expensive calls in WCategory::__construct()

Change-Id: Ie1bdbd8c24a6e108cbd1a76ba34635367b03e008