Refactor initialization of the email editor

This changes how the editor is initialized. The integration initialization class
doesn't get the engine class anymore, but integration is initialized via
mailpoet_email_editor_initialized action triggered from the engine.

This allows multiple integrations to hook into the engine.
[MAILPOET-5540]

mailpoet/tests/integration/EmailEditor/Integration/MailPoet/EmailEditorTest.php

@@ -27,6 +27,7 @@ class EmailEditorTest extends \MailPoetTest {
 
   public function testItRegistersMailPoetEmailPostType() {
     $this->emailEditor->initialize();
+    $this->diContainer->get(\MailPoet\EmailEditor\Engine\EmailEditor::class)->initialize();
     $postTypes = get_post_types();
     $this->assertArrayHasKey('mailpoet_email', $postTypes);
   }