[server] Introduce mapped() signal

Summary:
In KWin, we need to know when a sub-surface becomes mapped or unmapped
so we can generate or filter out window quads for the sub-surface.

Reviewers: #kwin, davidedmundson, apol

Reviewed By: #kwin, davidedmundson, apol

Differential Revision: https://phabricator.kde.org/D29256