Enforce string type for condition and action IDs

Problem/Motivation

Writing valid Rules expressions for reaction rules and components is difficult for new Rules developers, and some expressions that execute correctly have defects that will prevent them from being saved as configuration entities (an example: #2610574: Nested expressions throw exceptions when saved as reaction rule). When these kinds of data problems get into expressions, it can be difficult to find the source of the problem, even when using a source debugger.

While documentation and good examples will help here, it's also good if bad code calls attention to itself. In #2610574: Nested expressions throw exceptions when saved as reaction rule, it turned out that calls that should take a string plugin ID like RuleInterface::addCondition() and RuleInterface::addAction() were accepting expression objects. In these cases, the right API to use is ExpressionContainerInterface::addExpressionObject(). The code would execute correctly, but in both cases would throw an exception only when the developer tried to save the expressions as configuration entities.

Proposed resolution

Throw exceptions as close to the point of the errors as possible for PHP 5.x, and consider using scalar hinting and assertions once PHP 7 becomes common, to the extent that this kind of code is compatible with PHP 5.5.

Remaining tasks

User interface changes

API changes

No API changes for now, simply make sure that parameter types are better enforced.

Data model changes

None.