Create convenience functions for COM/WRL handling

There is a lot of verbose COM handling in qtbase in WinRT code paths. We can simplify and make these calls more consistent by making some macros or inline functions in qfunctions_winrt.h:

• QString/HString conversion functions. These normally take 3-4 lines. An inline function or macro could make it a one-liner. See "qt_QStringFromHString" in qnativesocketengine_winrt.cpp for inspiration.

• Print qErrnoWarning and return when an HRESULT fails. For example:
  • RETURN_VOID_IF_FAILED() // used inside constructors and functions which return void
  • RETURN_FALSE_IF_FAILED() // used inside methods that return bool
  • RETURN_HR_IF_FAILED() // used inside callbacks
  • RETURN_OK_IF_FAILED() // used inside callbacks that have side effects

• Global getters for the Core Window/Dispatcher might also be useful. This is done in several places (to great verbosity) and could be centralized into one location.
  • HRESULT getCoreWindow(ICoreWindow **);
  • HRESULT getCoreDispatcher(ICoreDispatcher **);