Mir 0.1.8

- mirclient ABI unchanged, still at 7. Clients do not need rebuilding.
- mirserver ABI bumped to 18. Shells need rebuilding.
- Server API changes affecting shells:
  . GLRenderer::tessellate() changed syntax.
  . graphics::Platform::create_display() has a new parameter allowing you
    to customize the compositor's (E)GL configutation.
  . Renderable::buffer(unsigned long frameno) is now:
    Renderable::buffer(void const* user_id). See below.
  . Renderable::should_be_rendered_in() is replaced by a more natural:
    Renderable::visble()
  . input::Surface::name() returns by value instead of reference now,
    to ensure future thread safety.
- Switched EventHub device enumeration and hotplug to Udev. NOTE! This
  means mir_test_* can't run natively on touch devices any more without
  some setup first:
    sudo mount -o remount,rw /
    sudo apt-get update
    sudo apt-get install -y umockdev
    umockdev-run -- bin/mir_unit_tests
- Added logging for HWC events.
- Continued consolidation of Surface classes toward a simpler architecture.
- Introduced "RenderableList" as the way to sample the Scene contents,
  and started using that in the default compositor.
- Introduced physical length units and conversion (geometry::Length) in
  preparation for arbitrary DPI rendering.
- Added some decorations to demo-shell; shadows and basic title bars, all
  anti-aliased and high-DPI scalable.
- Multi-monitor frame sync has been redesigned to eliminate the need for
  frame number tracking.
- Bugs (and enhancements) resolved; see below.