Mir 0.25.0

- ABI summary:
  . mirclient ABI unchanged at 9
  . mirserver ABI bumped to 42
  . mircommon ABI bumped to 7
  . mirplatform ABI bumped to 14
  . mirprotobuf ABI unchanged at 3
  . mirplatformgraphics ABI bumped to 11
  . mirclientplatform ABI unchanged at 5
  . mirinputplatform ABI bumped to 6
  . mircore ABI added, at version 1
- Enhancements:
  . Added pointer confinement support
  . Identify Raspberry Pi output types correctly
  . Added dead key and compose key support
  . Added gamma support for KMS hardware
  . Added yakkety and zesty gcc-6 support
  . Added surface passthrough support for fullscreen clients of nested
    servers
  . Introduced a new library `libmircore'
  . Began migrating from Protobuf to Cap'n Proto starting with events
  . Began support for VirtualBox (servers now start instead of crashing
    but full support for GL clients is still missing)
  . Began support for high precision frame timing, implemented for the
    KMS, Android and X11 backends so far
  . Added --display-report support for the X11 backend
  . Improved reliability of `mirvanity' latency test tool
  . Moved to using the CSS cursor naming scheme internally, although
    the old symbolic indentifiers remain unchanged and still work
  . Added refresh rate to MirSurfaceOutputEvent
  . Many improvements to automated test quality and coverage
  . Smoother desktop zoom using mir_proving_server (Super + two fingers)
- Bugs fixed: See below.