Add PictureViewer
Refactor theme system.
Module theme.py records current theme and provide interface to get and
set theme.
Moduel theme.py move from app.widget to app
Add: TweetWidget: cursor hover on avatar and thumbnail
Rename theme.py to theme_manager.py
Refactor account system.
Add module account_manager.py to manage current account.
Implemented: PictureViewer