Property operator

A new "prop" operator has been added to expressions so entities' properties can be accessed in expressions through Accessor. It works as follows:

( prop Object s"fully qualified id accessor understands" )

prop will use accessor to retrieve field defined in second operator from object defined in first operator.

Example:

"( prop $this s\"components<es.eucm.ead.engine.components.TagsComponent>.tags[0]\" )"

Returns the first tag in entity "$this"

This PR is not a priority, but will be needed for upcoming PRs