LibWeb+WebContent: Add abstraction layer for generic font families

Instead of hard-coding the names of system fonts to use for the CSS
generic fonts (like "sans-serif", "monospace", etc.) we now call out
to a Platform::FontPlugin and ask for the generic names.

Userland/Services/WebContent/CMakeLists.txt

@@ -11,6 +11,7 @@ set(SOURCES
     ConnectionFromClient.cpp
     ConsoleGlobalObject.cpp
     EventLoopPluginSerenity.cpp
+    FontPluginSerenity.cpp
     PageHost.cpp
     TimerSerenity.cpp
     WebContentClientEndpoint.h