Mir 0.8.0

- Enhancements:
  . Less sensitivity to ABI breaks - many headers unused by external
    projects are now hidden and not installed by -dev packaes. If you
    require any headers that are missing, just ask.
  . Touchspots: --enable-touchspots to servers; visually shows touch
    locations (warning: This affects performance LP: #1373692).
  . Client performance reporting: Any Mir client can now get accurate
    performance information (frame rate, render time, buffer lag etc)
    logged to stdout. Just set env MIR_CLIENT_PERF_REPORT=log
  . Further improved touch responsiveness, with less lag and smoother
    scrolling (so long as you don't enable touchspots).
  . Slightly faster builds using precompiled headers.
  . Turn hardware overlays on by default. When in use, this halves the
    CPU usage of a Mir server. Already enabled in unity-system-compositor.
  . More scripting to detect ABI breaks.
  . Improved src/ tree consistency (renamed "src/shared" to "src/common").
  . Improved fatal signal design: Changed from SIGTERM to SIGHUP delivered
    to clients on unexpected server disconnection.
  . Improved library/package design to allow concurrent installations
    of different Mir versions without conflicts.
  . Fd reception code is now common to client and server.
- ABI summary: Servers need rebuilding, but clients do not;
  . Mirclient ABI unchanged at 8
  . Mircommon ABI bumped to 2
  . Mirplatform ABI bumped to 3
  . Mirserver ABI bumped to 26
- API changes between Mir 0.7 and 0.8:
  . Lots of headers removed from the public SDK! We have only hidden
    headers not known to be used by any known projects. Please let us
    know if anything is missing - https://bugs.launchpad.net/mir/+filebug
  . graphics::Platform - interface changed significantly.
  . Lots of server API changes to support touchspots.
  . File descriptors now passed as type Fd instead of int32_t.
- Bug fixes: Lots. See below.