[4.0] Add hasField method to the JTable interface

[wilsonge]

Adds a hasField method to the JTable interface so we can phase out the use of property_exists in the system. This is in anticipation of the introduction of Joomla's entity system being introduced

This will require documentation on the b/c break to the interface (requiring a new method)

// cc @nikosdion as fof uses this interface (you already implement this method with the same signature - so no changes will be required) but as an FYI)

This work comes from the GSOC 2018 project by @isacandrei for webservices

[nikosdion]

Thank you @wilsonge :)