Added class for storing string literals.

Having a class for storing string literals rather than just having them
appear all over the place in code simplifies changing values (whether
because of a design change or because of a translation) and minimizes
the chance of user error (ie: String.Equals(element, Common.STR) versus
String.Equals(element, "STR") )

While this might be a bit more work, I think using this class will
greatly improve readability and manageability.