Move EventLoop.FdMask to module level

In my opinion, the ``FdMask`` enum nested in ``EventLoop`` makes no sense, but only complicates its use.


```python

class EventLoop:
    # […]
    class FdMask(enum.IntFlag):
        WL_EVENT_READABLE = lib.WL_EVENT_READABLE
        # […]
```


Suggestion: Move the class to module level