Rework class naming

[pfrenssen]

The project has adopted the practice from Symfony to prefix abstract classes with Abstract... but this has no value outside of the Symfony ecosystem and has a number of drawbacks:

• It is confusing for non-Symfony developers.
• It is providing redundant information.
• It causes related files to no longer be grouped together in directory listings.

Let's rename them to follow this standard:

• If the abstract class is intended as a starting point for a group of classes that implement a certain interface then we consider this as a "base class" and we add the suffix ...Base.
• If an abstract class is just intended to group reusable code shared by a number of subclasses, but is not intended as a general starting point then no prefix or suffix is added.