v1.4.0

ABI summary:

• mirserver ABI bumped to 50
  ###Enhancements:
• [MirAL] Support for exclusive zones
• [Wayland] Many Layer Shell improvements
• [mirclient] Make the mirclient socket optional and disabled by default
• [mir-utils] Delete mirrun and mirbacklight
• [mir-demos] Replace using miral-wayland in the scripts with a name of the form wayland-[0-9]

Bugs fixed:

• [Wayland] To accommodate GTK3 publish bespoke extensions before wl_seat. (Fixes #922)
• [Wayland] Handle mir_event_type_close_window. (Fixes #946)
• [Wayland] Send wl_buffer.release on the Wayland thread. (Fixes #951)
• [wlcs test fixture] Fix command-line processing so that --wayland-extensions can be used. (Fixes #906)
• Add header that needs to be explicitly included for protobuf 3.8. (Fixes #913)
• Track registering miral::WaylandExtensions objects with mir::Servers and error if there is more than one. (Fixes #875)