LibWebView+Ladybird: Begin de-duplicate WebView implementations

This starts moving code equally shared between the OOPWV and Ladybird
WebContentView implementations to WebView::ViewImplementation, beginning
with the client state.

Userland/Libraries/LibWebView/CMakeLists.txt

@@ -4,6 +4,7 @@ set(SOURCES
     OutOfProcessWebView.cpp
     RequestServerAdapter.cpp
     StylePropertiesModel.cpp
+    ViewImplementation.cpp
     WebContentClient.cpp
     WebSocketClientAdapter.cpp
 )