uxtheme.dll from Windows XP/2003/Vista/7/etc needs user32!IsServerSideWindow()

During my investigations with Windows components replacement, I have found out that Microsoft version of uxtheme.dll from Windows XP/2003/Vista/7/etc. needs IsServerSideWindow() function implemented in our user32.dll. It spams a lot about that function in the log at least, when testing Windows uxtheme.dll in ReactOS.

I have just imlemented it together with an appropriate apitest locally and will submit a PR soon. My implementation is based on the following unofficial documentation: http://undoc.airesoft.co.uk/user32.dll/IsServerSideWindow.php. This function is not offically documented by Microsoft, but fortunately it has that unoffical documentation.