Multiple event loops/backends (X11 and Wayland)

### Description Hello! I am developing an audio app that supports plugins. Most plugin formats support X11, and recently starting to support Wayland as well, and my main application windows will support either. I'm trying to figure out a way to have 2 event loops running at once to support the two windowing systems, using `mio`/`calloop` to dispatch between them. It seems there's currently (in `1.30`) a lock that prevents multiple loops to be created. Only on the web the lock can be dropped. It seems that beyond this, there shouldn't be any problem with having multiple loops on some platforms, at least unix. Can you allow having multiple loops running (maybe with somthing like `allow_event_loop_recreation`)? Or some other solution to support this use case? ### Relevant platforms Wayland, X11