Add a quit shortcut to the main window (#200)

The shortcut is Ctrl+Q on every platform but windows, which doesn have one.
See also: http://qt-project.org/doc/qt-5/qkeysequence.html#details (StandardKey Quit)

gui/MainWindow.cpp

@@ -35,6 +35,7 @@
 #include <QToolButton>
 #include <QWidgetAction>
 #include <QProgressDialog>
+#include <QShortcut>
 
 #include "osutils.h"
 #include "userutils.h"
@@ -116,6 +117,12 @@ MainWindow::MainWindow(QWidget *parent) : QMainWindow(parent), ui(new Ui::MainWi
 	// OSX magic.
 	// setUnifiedTitleAndToolBarOnMac(true);
 
+	// Global shortcuts
+	{
+		auto q = new QShortcut(QKeySequence::Quit, this);
+		connect(q, &QShortcut::activated, qApp, &QCoreApplication::quit);
+	}
+
 	// The instance action toolbar customizations
 	{
 		// disabled until we have an instance selected