In some cases QApplication::aboutToQuit() and
QApplication::commitDataRequest() signals not emitted on
Windows. This happened when qBittorrent is started with both "Show
minimized" and "Minimize to system tray" enabled.
All MainWindow child widgets use constructors with 'parent' parameter
that allows parent widget to delete them at its destruction.
Some other MainWindow cleanup code replaced to destructor.
Application manages MainWindow instance and remove it when aboutToQuit()
signal emitted.
Application process message and split it into params list itself (this
prevents code duplication).
Application store params unless other components ready to process them.
Application incapsulate all QMacApplication and SessionApplication logic
(this is too small to have separate classes).
Change project directory structure according to application structure.
Change 'nox' configuration option to something more meaningful 'nogui'.
Rename 'Icons' folder to 'icons' (similar to other folders).
Partially add 'nowebui' option support.
Remove QConf project file.
