Make WinExtra modul platform-independent

We are currently using WinExtra to show progress in taskbar under Windows. But implementation with WinExtra doesn't look really much better than using WinAPI direct, because we still need a lot of #ifdef windows in our source and in CMake.

So it would be nice, if this module would platform-independend:

• WinExtra available under Linux, Mac, ... too, so we don't need if(Win32) in CMake
• Make QWinTaskbarProgress etc. available on all platforms, so we don't need #ifdef in our source.

I know QWinTaskbarProgress is platform specific to Windows, but from view of developer it is just a ProgressBar. Why not implement a (invisible) dummy progressbar on all other systems, so the code is compiling under all platforms and running without crashes, even it does nothing?