Remove unnecessary filtering of key events in PopupWindow

In this way Tab key (and other focus movement keys) can be used
in PopupWindows. They aren’t propagated to the main window because
a correct filter was added in 883629b563

src/ui/popup_window.cpp

@@ -97,11 +97,6 @@ bool PopupWindow::onProcessMessage(Message* msg)
           closeWindow(NULL);
         }
 
-        // If we are filtering messages we don't propagate key-events
-        // to other widgets. As we're a popup window and we're
-        // filtering messages, the user shouldn't be able to start
-        // other actions pressing keyboard shortcuts.
-        return false;
       }
       break;