Make the class variables protected for Migration

Migration class (from the migrate_drupal module not from the migrate module) variables should not be accessed directly. Functions should be use to access the variable. For instance use getDescription() and setDescription($description) for the protected class variable description. For a boolean variable the getter function becomes isVariableName(). In object-oriented programming this is called encapsulation.

Remaining tasks

• Update the class variables and make them protected.
• Create getters and setters for frequently used get and set functionality.
• Update drupal to use the getters and setters instead of accessing variables directly.
• There are no tests required because the added functions are only getters and setters.

For more info over what should be done see the issue summary of #2016679: Expand Entity Type interfaces to provide methods, protect the properties.

Beta phase evaluation

But impact will be greater than the disruption, so it is allowed in the beta.