Cope with application-less windows

During the destruction phase, when the main window is destroyed, we might still get notifications and signals from the player object while we wait for the window instance to disappear. If that happens, the application's backreference on the window is going to be None, which means we can't get the AudioPlayer instance out of it. In order to cope with that, we can make Window::player() return an Option value, and update the call sites to cope with that.

Fixes: #317 (closed), #319 (closed)