Add xdg-decoration-unstable-v1 support
This is a redo of #638, rebased against master and updated to use the latest v6 proposal (https://lists.freedesktop.org/archives/wayland-devel/2018-May/038206.html).
This adds support for xdg-toplevel-decoration in rootston. The preferred mode is "client". rootston will accept the client's mode requests.
A new example client is added. When launched, it will be yellow if using server-side decorations and green if using client-side decorations. The client's initial preferred mode can be given as an argument (either "client" or "server"). The client's preferred mode can be toggled by clicking on its surface.
Test plan: try the example
Follow-up refactoring: use the new configure
and ack_configure
events for xdg-toplevel and xdg-popup.