Add ThemeUninstallValidatorInterface similar to ModuleUninstallValidatorInterface

Problem/Motivation

ModuleUninstallValidatorInterface services are the recommended way (see https://www.drupal.org/node/2392677 → ) to prevent uninstalling a module if there are e.g. config dependencies that should avoid uninstalling the module.

However, we lack something similar for themes, while e.g. config can depend on themes and we might want to prevent uninstalling them.

Proposed resolution

Add ThemeUninstallValidatorInterface similar to ModuleUninstallValidatorInterface.
Evaluate if we can refactor ModuleUninstallValidatorInterface to use a common ExtensionUninstallValidatorInterface.

Remaining tasks

TBD

User interface changes

TBD

Introduced terminology

TBD

API changes

TBD

Data model changes

TBD

Release notes snippet

TBD