Bug 54726 - Add Windows unicode cmd argument support

Instead of working with the encoded sys.argv, we now use the unicode
pywikibot.argvu, which is set by the active userinterface. The UI
knows how to get the correct unicode argv (depending on OS).

pwb.py is adapted to use pwb.argvu in addition to sys.argv

Change-Id: Ifde7e42587f4a4493ca3c5d8f5ed1ab06bbd806b
Merge "Bug 54726 - Add Windows unicode cmd argument support"