Some parts of Theme\Registry are written as if they support multiple themes

Let's see if this breaks anything.

Locally this is a branch with various distinct commits:

* 9af751c (HEAD -> Theme-Registry-2957451-8.6.x) Issue #2869859: Stop pretending that Theme\Registry is for more than one theme.
* f6079ee (CS+) Initialize Registry->runtimeCache so that it is never NULL.
* ea601c9 (CS) Split arguments to multiple lines.
* 656110d (CS) Break constructor arguments to multiple lines.
* 6fde882 (CS) Document that Registry->runtimeCache can be NULL.
* 0695b1b (CS) Add inline @var doc for result of array_reverse().
* 7344691 (CS+) Use === when comparing with strings.
* d00af06 (CS+) Add third parameter TRUE in in_array().
* f8676ba (CS+) Use str_replace() instead of strtr().
* c28df38 (CS) Use single quotes.
* f45028b (CS+) Add (previously undeclared) property Theme\Registry::$themeInitialization.

EDIT: This will fail.

In #3 I forgot half of the required changes.
Trying again.

Trying a patch which has only the CS changes and not the functional changes.

I just notice that Registry->initialized is never set to TRUE in init()!!
Ouch.
This is a bug in 8.6.x.

I just notice that Registry->initialized is never set to TRUE in init()!!

The effect is:
Every call To ThemeManager->render() calls Registry->getRuntime()
which calls Registry->init()
which calls Registry->themeManager->getActiveTheme()

This means that, as soon as themeManager->activeTheme changes, ThemeManager->render() will render in the new active theme.
We could say this is great, it means theming functionality is always up to date.
But at what cost?
We repeat the same redundant operation each time we execute a theme hook. This is not right.

Testing my hypothesis about ->initialized.

#12/#13 shows:

• Drupal\Core\Theme\Registry::$initialized always stays FALSE.
• The check for $this->initialized in Registry::init() is pointless.
• In the current implementation, Registry must call $this->themeManager->getActiveTheme() repeatedly.
  We can assume that this has a performance impact.

Some clarification:

The problem here is NOT the "theme system" itself with its "theme hooks", which some consider archaic and want to modernize.
You can think what you want about this, but it is not the problem here.

The real problem here is how different components deal with the possibility that the "active theme" can change mid-request.
So, it is a question of how information and state travels in our architecture, the order in which stuff happens in bootstrap, how per-theme logic is organized.

This needs to be cleaned up, no matter if we switch to plugins or whatever else has been suggested afterwards.

This issue is not "Needs review". The patches were just to see what would happen.