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/CMakeLists.txt

@@ -50,6 +50,7 @@ set(SOURCES
     ConsoleGlobalObject.cpp
     CookieJar.cpp
     EventLoopPluginQt.cpp
+    FontPluginQt.cpp
     RequestManagerQt.cpp
     main.cpp
     WebView.cpp