Refactor font family rendering using CSS inlining

We don't reset font family on any level, so there is no need to
bubble the setting using a preprocessor and render the inline styles
explicitly in every block.

In this commit, I change how font-family settings are distributed/rendered
in the email renderer. In the new approach, we rely on class names defining font-family
and a generated CSS sheet with font-family definitions.
We apply the font-family CSS by inlining CSS rules for families in the later phase of
rendering after all individual blocks are processed.
[MAILPOET-5740]

mailpoet/tests/integration/EmailEditor/Engine/SettingsControllerTest.php

@@ -0,0 +1,19 @@
+<?php declare(strict_types = 1);
+
+namespace MailPoet\EmailEditor\Engine;
+
+class SettingsControllerTest extends \MailPoetTest {
+  /** @var SettingsController */
+  private $settingsController;
+
+  public function _before() {
+    parent::_before();
+    $this->settingsController = $this->diContainer->get(SettingsController::class);
+  }
+
+  public function testItGeneratesCssStylesForThemeWithFontFamilies() {
+    $css = $this->settingsController->getStylesheetForRendering();
+    verify($css)->stringContainsString('.has-system-sans-serif-font-family');
+    verify($css)->stringContainsString('.has-system-Serif-font-family');
+  }
+}