New shell plugin for single-app usecases

To make life easier for people shipping embedded devices, and reduce the proliferation of desktop-shell hacks, it would be great to have a single-purpose shell which would pretty much just run a single client.

This shell would:

• offer no UI, chrome, decor, or furniture, of any kind
• start a single client named in the config file, either restarting it or shutting down when the client went away
• make all top-level surfaces fullscreen
• attach dialogs to their parents
• raise and focus any new top-level/dialog surface created
• support click⁰-to-move and click-to-raise

I don't think it needs much anything more complex than that.

This would also serve as a useful example of how to drive Weston's window management.

⁰: 'Click' means both pointer buttons as well as touch.