Made 'set_level' available from WindowDesc

[Schaback]

There is no simple way of setting the WindowLevel prior to creation for a window.
This would allows tiling window managers like i3 to see the _NET_WM_WINDOW_TYPE_DIALOG flag on window creation
and mark the window as floating.
Since all other options of WindowConfig are exposed in WindowDesc, this seems like an oversight. :)

[cmyr]

This seems reasonable to me, thanks!

I dont' really like us using the prefix set_ on these methods since it implies mutation and these are builder methods; but these particular methods are all over the place, so I think it's fine for now, and at some point in the future we can do a big 'api consistency' pass and fix this. 🤷

[Schaback]

Thanks! Yea, same. In general i found it confusing as a noob, how the WindowConfig was supposed to be used, or rather why this layer of pass-trough with WindowDesc - hides -> WindowConfig was there. This interface could def. use some reworking when api consistency is looked at.