Standalone library for client side window decorations

Some wayland client examples (weston-terminal, weston-editor, ...) use a simple toolkit (toy toolkit) to render client side window decorations.

Similar to libweston, it would be useful to separate this window decoration functionality into a dedicated library. This library could then be used by other wayland clients to render client side decorations, in case these projects do not need an actual tookit (like Qt or Gtk) and do not want to write platform-specific code for window decorations on Linux. There are plenty of projects like SDL and Kodi that do not make use of toolkits and need to implement their own decorations.

The use of such a simple client-side-decoration library has been discussed in a Gnome Mutter issue: https://gitlab.gnome.org/GNOME/mutter/issues/217#note_553315 and a prototypical plugin-based implementation of libdecoration can be found at https://gitlab.gnome.org/jadahl/libdecoration.

It would be great if the Weston project could provide such a minimalist library (and use it for its example clients).