Provide default constructors for UIA operation abstraction types (where it makes sense)

[LukaszMakar]

To simplify code built with the UIA operation abstraction library, we should consider simplifying construction of the types provided by the library.

Specifically, types whose default values are implied or types that in other languages are often default-constructible could be good candidates, such as UiaString that right now has to be constructed as UiaString{ L"" } while the C++ equivalent of std::wstring can be constructed without providing arguments.