Add Ctrl+F shortcut for quickly enabling/disabling channel filtering
Default enable hide empty channel behavior for filter

- This should enable a discoverable first-time user experience
  with the filter and be usable without specifically selecting
  channels to filter. Using it in combination with the shortcut
  is quite handy even on small severs with many empty channels.
Expand "What's this" and tooltip for filter action
Make sure filter never hides the channel the user resides in.

- This only fixes the bug and doesn't perform the needed
  refactoring in the implementation of the channel filtering
  logic.