LibWeb: Add a basic content filter (ad blocking!) :^)

This patch adds a global (per-process) filter list to LibWeb that is
used to filter all outgoing resource load requests.

Basically we check the URL against a list of filter patterns and if
it's a match for any one of them, we immediately fail the load.

The filter list is a simple text file:

    ~/.config/BrowserContentFilters.txt

It's one filter per line and they are simple glob filters for now,
with implicit asterisks (*) at the start and end of the line.

Libraries/LibWeb/CMakeLists.txt

@@ -177,6 +177,7 @@ set(SOURCES
     Layout/TreeBuilder.cpp
     Layout/WidgetBox.cpp
     LayoutTreeModel.cpp
+    Loader/ContentFilter.cpp
     Loader/FrameLoader.cpp
     Loader/ImageLoader.cpp
     Loader/ImageResource.cpp