Remove magic property overloading from Extension class

#2228093: Modernize theme initialization → originally intended to eliminate the magic overloading of theme info properties on Extension objects, but did not.

The magically overloaded theme extension properties are still used:

Undefined property: Drupal\Core\Extension\Extension::$info

Drupal\Core\Extension\ThemeHandler->addTheme(Object)
Drupal\Core\Extension\ThemeHandler->listInfo()
list_themes()
_drupal_maintenance_theme()
drupal_maintenance_theme()
drupal_flush_all_caches()

The reason is:

#2228093: Modernize theme initialization → only cleaned up theme initialization.

However, ThemeHandler::rebuildThemeData() is unrelated to initialization and still operates on Extension objects only.

Given that there is a use-case for instantiating a Theme domain object (including base themes) within that process, we should probably rename the ActiveTheme class into just Theme. — No change in behavior. It only doesn't have to be the currently active theme, you can instantiate it for any theme.