Added channel hiding/filter feature

* To enhance usability of mumble on servers with lots of channels
(like mumble.piratenpartei-nrw.de)
Reduce variable scopes
Move method comments. Remove obsolete.

* Move method comments to the actual methods.
* Remove obsolete comment
that came in in aad4de178bc5cac60cdf353934379d0acc053be3
The signal/slot `activated` is no longer used since a long time ago.
Unify comment style.

* Adjust comments to unified JavaDoc-style comments
Channel Filtering: Fix shortcut. Specific text.

* Fix/implement the bindable global shortcut for marking a channel hidden when filtering.
* Make text more specific to prevent misunderstanding.
Rename variable. Informative tooltips.

* Rename variables to match its function
gsMetaChannel -> gsJoinChannel
gsMetaLink -> gsLinkChannel
* Add tooltips to shortcuts to reference necessary whisper-to.
Add shortcut for de-/activating channel filter