Wayland client abstraction

[wmww]

We have various pieces of Wayland client and rendering logic scattered around our code bases:

• Mir examples
  • Background decoration creates a window and renders text in software
  • Spinner creates a window and renders with EGL
  • Kiosk creates a window and renders in software
• Internal SSD renders text in software and handles input
• Egmde creates widows and renders various shell components which include text

While some users of Mir may find it useful to use a full toolkit such as GTK or Qt, we don't want to depend on or show bias towards a large toolkit, we don't need that much complexity and we would like more flexability as to what protocols we support. In particular using layer shell may allow us to reduce shell complexity by letting shell components position themselves.

It may make sense to make internal helpers, an external library, or pull in a 3rd party library (such a GLFW)

[wmww]

Related to #1390

[AlanGriffiths]

Another example is the Mir-on-Wayland platform. Like several examples listed above this creates a not only a client but also fullscreen surfaces.