Plain entities cannot be used at this point as root in FormHelper

Trying to do so was throwing exceptions because it tried to find a table
called 'entities'. Disallowing this usage until we find a better
solution
Marking entities with information containing the table they come from
More work towards marking entities with their source
Fixing some failing tests
Marking entities on save and fixing more tests
Unit testing Entity after adding the source() method
Adding test to Marshaller for showing that it adds the entity source
Small refactoring
Making EntityContext more intelligent by reading the entity source
Fixed failing test that was brought from 2.5 code
Making Entity::source() just a string
Fixing some refactoring artifacts
Merge pull request #3030 from cakephp/3.0-entity-context-fixes

3.0 entity context fixes