Make sure shortcuts work all the time by adding all QAction with shortcuts to main window's widget

The shortcuts (such as Cltr+W, Cltr+Q, etc) not always work (at least on Ubuntu 14.10 with Qt5). This behavior is observed when QAction isn't added to a widget that's always listening for events (such as the main window). This patch adds every QAction with shortcut to main window's widget, making sure it works all the time.