First set of fixes for filter patch

Compare to review comments on https://github.com/mumble-voip/mumble/commit/304bf438daecad36285b8647b9bc72b685cf31ca

- No longer use channel name for identifying filtered channels.
  Switched to using server certificate digest combined with Channel
  id which works between servers and with duplicate channel names.
  The digest should be replaced by proper server identification soon.
  Ideally the client should keep a list which identifies servers
  by their address or - if a strong certificate is available - by
  certificate email. This can be re-used for tokens and other server
  dependent configuration.
- Changed naming from 'hidden' to 'filtered'
- Removed tray menu entry
- Removed global shortcut for channel hiding/showing