Remove channel filtering from tray
Disconnect if for some reason we established a connection to a server without it having a certificate
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
Fix channel flag handling with filter patch

- Channel comment flags were no longer clickable because
  filters weren't considered in offset calculation.
- What's this text didn't contain the flag icon
- Lowercased Flag.svg
- Magic numbers and abbreviation refactoring
- Lowercased Filter.svg and improved license display