Release 2.12.1

Bugs fixed:

• Add missing #include statements (#2802)
• mir_wayland_generator generated code that include non public
  header (#2804)
• Building docs fails when trying to not remove doc/html/cppguide (#2800)
• wayland: Add missing cstdint includes (#2821)
• Enabling zwp_idle_inhibit_manager_v1 causes Mir to crash with some
  applications (#2836)
• [Wayland] idle_inhibit_unstable_v1 is badly broken (#2838)
• A couple ShmBacking.* tests failing on PowerPC (#2774)
• Fix BasicIdleHub behavior (#2843)