Add explicit SurfaceInterface::commited signal

Summary:
Whilst some interfaces that are double-buffered against the wl_surface
are proxied through SurfaceInterface, some are not, most notably
XdgShell.

We need some low level signal to know when the surface has been
committed which doesn't rely on their being a damaged buffer.

Test Plan:
Unit test
Used in kwin

Reviewers: #kwin, zzag

Reviewed By: #kwin, zzag

Subscribers: zzag, kde-frameworks-devel

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D18582