Improvements to expressions and variables system

A couple of improvements in variables system and expressions required for other upcoming PR:
- A "$this" variable that points to the current entity holding the expression being parsed has been added. This required changing VarsContext and VariableDef a little
- A "HasTag" operator has been added. This new operator takes an entity ($this supported right now, only) as first argument and a string representing a tag as a second argument. It checks the entity actually contains the given tag.