Mir 0.27.0

- ABI summary:
  . mirclient ABI unchanged at 9
  . mirserver ABI bumped to 44
  . mircommon ABI unchanged at 7
  . mirplatform ABI bumped to 61
  . mirprotobuf ABI unchanged at 3
  . mirplatformgraphics ABI bumped to 13
  . mirclientplatform ABI unchanged at 5
  . mirinputplatform ABI bumped to 7
  . mircore ABI unchanged at 1
- Enhancements:
  . Mostly groundwork required to support major enhancements coming in
    future Mir versions.
  . Removed android-input and eliminated the entire "3rd_party/" subtree.
    Now the Mir source tree contains original code only.
  . Added mir_prompt_session_new_fds_for_prompt_providers_sync API.
  . mirout: Added load and save options for keeping display configs
    on disk.
  . mirout: Added "--" support for applying configuration changes under
    Unity8.
  . Fixed failure of DRM hardware cursor {hide(); show(image);}
  . Added server option: "--cursor software" (MIR_SERVER_CURSOR=software)
  . Added letterboxing/black bars support to the GL renderer in preparation
    for generic output cloning.
  . Added client API for getting the logical size of an output.
  . Migrated MirCookie to use SHA-256.
  . Ensure RealKMSOutputConfiguration stays in sync with actual hardware
    state.
  . Added support for drag-and-drop.
  . Lots of other client API enhancements.
  . Minor clean-ups, optimizations and dead code removal.
  . Added support for building on Ubuntu 17.10 artful.
  . Update example code to use undeprecated API.
  . mesa-kms: Support hardware cursors in hybrid setups.
  . Rework and publish the graphics platform APIs
- Bugs fixed: See below