StringViewize a bunch of things -- mostly LibGUI

Author: https://github.com/rburchell
Commit: 1024dfa81a
Pull-request: https://github.com/SerenityOS/serenity/pull/170
Reviewed-by: https://github.com/awesomekling

LibGUI/GDirectoryModel.cpp

@@ -272,7 +272,7 @@ void GDirectoryModel::update()
     did_update();
 }
 
-void GDirectoryModel::open(const String& a_path)
+void GDirectoryModel::open(const StringView& a_path)
 {
     FileSystemPath canonical_path(a_path);
     auto path = canonical_path.string();