LibGUI+Browser: Move GUI::UrlBox to the Browser application

Browser is the only user of this component. Move it to allow making use
of LibWebView for URL highlighting.

Userland/Applications/Browser/CMakeLists.txt

@@ -25,6 +25,7 @@ set(SOURCES
     StorageModel.cpp
     StorageWidget.cpp
     Tab.cpp
+    URLBox.cpp
     WindowActions.cpp
     main.cpp
 )