[wayland] Finish initialising ShellClient only when commited to the surface

Summary:
Everything on the wl_surface is double buffered.

When we create an XdgShell toplevel or popup we shouldn't treat it as
attached until it's committed to the surface.

A client should commit the surface after it's sent it's initial state of
the Xdg topLevel; minimumSize, title, app_id, etc.

By blocking sending configure events we will have flushed the correct
initial state before sending a single atomic correct event to the
client. It also adds a hook to re-evaluate rules now that all properties
are set.

Arguably this applies to WlShellSurface too, but I've left it unchanged
as it's deprecated and hard to verify real client behaviour.

Test Plan: Ran all unit tests

Reviewers: KWin, zzag

Reviewed By: KWin, zzag

Subscribers: zzag, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D18583