WL: flush display before managing new window
WL: flush display before managing new window

This is needed as clients might not be ready to be mapped when they are
created, and when we begin managing new windows we try to focus them and
map them. In the x11 backend this isn't an issue because we only begin
managing them at their first map, rather than their creation, but we
need to handle their creation in the Wayland backend so we can listen to
their events (including the map event). Mapping when clients are not
ready results in segfaults when their surface is accessed when it should
not be. Flushing the event queue after subscribing listeners but before
managing (and focussing) the window gives the window a chance to
continue setup and get ready for mapping.

qtile/qtile,qtile/qtile,ramnes/qtile,ramnes/qtile