Use application-level event filter for handling shortcut redirects

* If we just filter the main window, it won't properly pick up shortcuts
  used in floating windows.

qrenderdoc/Windows/MainWindow.cpp

@@ -67,7 +67,7 @@ MainWindow::MainWindow(ICaptureContext &ctx) : QMainWindow(NULL), ui(new Ui::Mai
 
   QToolTip::setPalette(palette());
 
-  installEventFilter(this);
+  qApp->installEventFilter(this);
 
   setAcceptDrops(true);