[USER32] Implement IsServerSideWindow (undocumented)

Implement IsServerSideWindow function in user32. It checks whether a window has a window procedure that resides in kernel mode and returns a boolean condition based on the appropriate internal window state flag (WNDS_SERVERSIDEWINDOWPROC).
This function is not officially documented by Microsoft, but fortunately it has unofficial documentation at http://undoc.airesoft.co.uk/user32.dll/IsServerSideWindow.php.
Required and used internally by uxtheme.dll from Windows XP/2003/Vista/7/etc. The log spams with it a lot, when using uxtheme.dll from Windows in ReactOS.
CORE-19946

win32ss/user/user32/misc/stubs.c

@@ -432,15 +432,6 @@ WORD WINAPI InitializeWin32EntryTable(UCHAR* EntryTablePlus0x1000)
   return FALSE;
 }
 
-/*
- * @unimplemented
- */
-BOOL WINAPI IsServerSideWindow(HWND wnd)
-{
-  UNIMPLEMENTED;
-  return FALSE;
-}
-
 /*
  * @unimplemented
  */

win32ss/user/user32/windows/window.c

@@ -1512,6 +1512,37 @@ IsIconic(HWND hWnd)
 }
 
 
+/**
+ * @brief
+ * IsServerSideWindow
+ *
+ * Checks whether a window has a window procedure that resides in kernel mode.
+ *
+ * @param[in] hWnd
+ * A handle to the window to be checked.
+ *
+ * @return
+ * TRUE if the window has a window procedure that resides in kernel mode,
+ * FALSE otherwise.
+ *
+ * @remarks
+ * Undocumented, see http://undoc.airesoft.co.uk/user32.dll/IsServerSideWindow.php
+ * (unofficial documentation).
+ */
+BOOL
+WINAPI
+IsServerSideWindow(
+    _In_ HWND hWnd)
+{
+    PWND Wnd = ValidateHwnd(hWnd);
+
+    if (Wnd != NULL)
+        return (Wnd->state & WNDS_SERVERSIDEWINDOWPROC) != 0;
+
+    return FALSE;
+}
+
+
 /*
  * @implemented
  */