Added constant to retrieve Windows' Documents path

According to MSDN's
documentation (https://msdn.microsoft.com/en-us/library/windows/desktop/bb762181(v=vs.85).aspx),
CSIDL_PERSONAL represents the default path to Windows Document's folder:

"Previous to Version 6.0. The file system directory used to physically
store a user's common repository of documents. A typical path is
C:\Documents and Settings\username\My Documents. This should be
distinguished from the virtual My Documents folder in the namespace. To
access that virtual folder, use SHGetFolderLocation, which returns the
ITEMIDLIST for the virtual location, or refer to the technique described
in Managing the File System."

platform/windows/os_windows.cpp

@@ -2075,7 +2075,7 @@ String OS_Windows::get_system_dir(SystemDir p_dir) const {
 			id=CSIDL_MYPICTURES;
 		} break;
 		case SYSTEM_DIR_DOCUMENTS: {
-			id=0x000C;
+			id=CSIDL_PERSONAL;
 		} break;
 		case SYSTEM_DIR_DOWNLOADS: {
 			id=0x000C ;