Mir 0.1.7

- mirserver ABI bumped to 17. All shells will need rebuilding.
- mirclient ABI unchanged. Clients do not need rebuilding.
- Server API changes (AKA why doesn't my code build any more?):
  . Class "CompositingCriteria" has been removed. It's replaced by the more
    flexible "Renderable" interface. This also resulted in parameter
    changes for the Renderer and scene filtering classes.
  . The function "DisplayConfiguration::configure_output()" has been
    removed. Instead, please use the new mutable version of
    "DisplayConfiguration::for_each_output()" with which you can modify
    the output structure passed in on each iteration.
  . Exposed formerly private class "GLRenderer" and demonstrated how
    to override its behaviour in demo-shell. This area is under
    construction and may experience further major changes.
- Added initial support for hardware (HWC) overlays to accelerate
  rendering and reduce power consumption. Not complete yet.
- Screen rotation: Added mouse cursor rotation support, so you can now
  still control things on a rotated screen. Still missing rotation of
  the cursor bitmap itself.
- Lots of fixes to support nested Mir servers (see below).
- Major simplification to how surface size/position/transformation
  interact, making transformations much easier to manage and work with.
- Lots of bugs fixed (see below)