modify to support multiple grammars, add fmpxmllayout grammar

The approach taken was to convert the execute() method to route based on grammar and to move all of the parsing into grammar specific parsing methods. Also added an array property to validate that a supported grammar has been specified. fmxmlresult is the default, to support backward compatibility.
Yes, exactly. Everywhere that currently uses the string literal should just be refactored to use the constants, including outside the adapter. For example, an implementing class would be able to do `Adapter::FMPXMLLAYOUT_GRAMMAR` instead of `'FMPXMLLAYOUT'`.