Convert PluginUIManager to extend DefaultPluginManager

I'll get a patch out in a bit.

Hmm... the patch applies fine locally... I guess I'm missing something.

Aha this might work.

+++ b/core/modules/system/lib/Drupal/system/Plugin/Type/PluginUIManager.phpundefined
@@ -7,33 +7,35 @@
-    $this->discovery = new AlterDecorator($this->discovery, 'plugin_ui');
-    $this->discovery = new CacheDecorator($this->discovery, 'plugin_ui');
...
+    $this->alterInfo($module_handler, 'pluginUI_info');
+    $this->setCacheBackend($cache_backend, $language_manager, 'pluginUI');

Why rename the hook? Apart from camel case being wrong here anyway, it seems like an unrelated API change.

Bleh the line right above those got me into camelCase mode... :(

Here we go - I guess I wasn't synced when I made the patch even though I thought I was...

PluginUI is on its way out, so I believe we can close this: #2056513: Remove PluginUI subsystem, provide block plugins UI as one-off.