v0.15 release - February 2022

Note: existing machines will need to be reloaded, and might need to be
recreated after this update!

- AWS is now supported as a provider 👏
  - ready-made and publicly available boxes for Linux, Windows, and macOS
  - configure a security group that allows traffic only from the host's
    public IP address
  - key-based ssh authentication using the host user's key pair
  - optional auto-shutdown support to stop idle instances
  - with VNC GUI access via vnc-server role (needs to be added explicitly)
    to Linux
- base box maintanance 🔧
  - Windows 10 Azure image upgraded to 20H2
  - macOS 11 and 12 upgraded to latest available versions
  - keyboard navigation is now enabled on macOS, unit tests relying on
    focus changes by tabbing now pass
- provisioning improvements 🛠
  - android-emulator improvements
  - vnc-server role implemented for Linux using x11vnc
  - linux-desktop role falls back to Xvfb if there's no screen, enabling
    it on cloud instances
  - install provisioners are now merged correctly
- job improvements 🏃
  - pass the Qt host-tools path to projects when building on an Android
    builder box
  - update the qmake/qt-cmake wrapper scripts more frequently to make it
    easier to switch Qt versions
  - call the configure script explicitly using /bin/bash
- other plumbing 🐞
  - recreate mutagen sync when a machine's ssh port forwarding changes
  - fix the upgrade logic in the Windows installer script
  - old boxes are removed from the default minicoin.yml file
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