mixins cleanups

[Antoine Martin]

Follow up from #1761 + #1700.

Tasks:

• write more unit tests: #1773, also helpful for:
• identifying leakage of mixin domains: where one mixin relies on another (ie: mmap)
• messy code:
  • keyboard, ie: modifier_client_keycodes is still too ugly
  • hello packet code
  • init_ui
• r18656 for window server: remove all window packet handlers if window forwarding is disabled
• networkstate: pings require statistics object - move it out into its own domain?
• ServerCore.setup(): we init_auth and init_html_proxy with options, should be split into a parsing step + setup step (means losing support for the legacy password-file - and need another solution for socket_dir in sys-auth-base)
• more consistent namespace for mixin variables, try to distinguish: settings (command line, env vars), server settings (from hello or update packets), state (focus, buttons, etc)
• some timers can now be tracked and cancelled more easily (ie: ping timers)