Allow window-specific landing-menu actions even if no window is attached

We currently do not support running the landing-page specific menu-items if no audapolis window exists. On MacOS (only?) the application is not stopped if all windows are closed, so you can still access the menu and run actions from there. We only allow non-window-specific action (see #300/#301), but it might be nice to instead support all actions that can be performed from the landing page (Import, Open, New Blank Document).

With the current codebase this would require us to first open a new window, wait for it to load and then dispatch the right action in it. I think this is non-trivial right now for a couple of reasons:

1. The [actions are identified by uuids](https://github.com/audapolis/audapolis/blob/23998cd36b747cfda11f23609dc1bb2c074d6d53/app/src/components/MenuBar.tsx#L100-L104) which differ between all windows, so there currently is no way of dispatching these actions.
2. We do not have a way to check when a window is fully loaded

One way might be to keep a list of "stuff to do" in the main process which the window fetches after loading fully and then executes.

As our resident menu expert (😄 ), what are your thoughts on this @anuejn?