[policy, then patch] Decide on coding standard for configuration keys of ConfigEntities

Problem/Motivation

The member variables of ConfigEntity's map 1-to-1 to the configuration keys in the corresponding config YAML files. Coding standards dictate lowerCamelCase for member variables in OOP code and underscore_separated for config keys. We need to decide what we want here. Most ConfigEntity's are not bitten by this, because they only contain one-word keys, in which case lowerCamelCase and underscore_separated fall together. :-)

We already have an inconsistency in core with the View entity (View::base_table) and the Breakpoint and BreakpointGroup entities.

Proposed resolution

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Remaining tasks

Decide on one of the following:

1. Make class members of configuration entities that map to configuration keys underscore_separated.
   Con: This violates the coding standards for object-oriented code.
2. Make configuration keys that map to class members of configuration entities lowerCamelCase.
   Con: This violates the coding standard for configuration keys.
3. Change our coding standard to require lowerCamelCase for all configuration keys.
   Con: It feels unnatural to use lowerCamelCase for non-object-oriented code. (This in a way applies to 2. as well.)
4. Introduce an abstraction layer that maps underscore_separated configuration keys to lowerCamelCase class members.
   Con: More code, additional abstraction.

User interface changes

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API changes

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