Pass timeline explicitly to `TimelineUIs.on_timeline_command`

I would be like to be able to do something like:

```
from tilia.ui import commands

tl_id = commands.execute("timelines.add.marker", name="test")
commands.execute("timeline.marker.add", time=0, label="a marker", timeline=tl_id)
```

However, "TimelineUIs.on_timeline_command", the callback for the "timelines.add.*" command, does not take a `timeline` argument.