Make activated() signal compatible with Qt. Allow setting app_id via the factory() function
Re-enable the system tray icon on linux. System tray icons now work in any desktop environment that supports the StatusNotifier spec, such as Ubuntu Unity, KDE 4+, GNOME 3, etc.

Since calibre 2.0, the system tray icon functionality in linux had been
disabled because of bugs in Qt 5. calibre now contains its own system
tray icon implementation to work around Qt 5 bugs.