Ladybird: Add a FontPlugin and try much harder to find suitable fonts

Instead of only looking at the SerenityOS default fonts, we now also
look recursively in /usr/share/fonts for suitable fonts that we can
load and use.

Ladybird/FontPluginQt.h

@@ -0,0 +1,27 @@
+/*
+ * Copyright (c) 2022, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Vector.h>
+#include <LibWeb/Platform/FontPlugin.h>
+
+namespace Ladybird {
+
+class FontPluginQt final : public Web::Platform::FontPlugin {
+public:
+    FontPluginQt();
+    virtual ~FontPluginQt();
+
+    virtual String generic_font_name(Web::Platform::GenericFont) override;
+
+    void update_generic_fonts();
+
+private:
+    Vector<String> m_generic_font_names;
+};
+
+}