Implement global shortcuts

We want new functionality. QAction now does not able to implement global hot keys - i.e. hot keys which are handled even when application is not active.

Thi easily can be implemented under windows and linux. Possibly in Mac os X.
For linux: http://jaredforsyth.com/content/linux-global-hotkeys
For Windows: http://msdn.microsoft.com/en-us/library/ms646309(VS.85).aspx (RegisterHotKey function)

Qt::ShortcutContext should be extended and contain new item Qt::GlobalShortcut or Qt::DesktopShortcut