Add get_win32_portable_user_data_path()

As I talked with @normanjaeckel yesterday, I added a new function in main.py to return correct user data path ("/openslides") of portable version. Now, all openslides specific data (settings, database, gui-settings-file, whoosh-index - and later the plugins dir) are in the "openslides" dir in the portable root diretory.

This should be used by #1178.