Textual now has a "Mute Sound" menu item inside of the File menu.

• Abstracted the mute toggling into IRCWorld.m, away from IRCClient.m where it doesn't appear to belong.
• "File -> ⇧⌘M Mute Sound" toggles mute
• Planning on added an a dictionary item for the defaults system for this setting.
Merge branch 'master' of https://github.com/winny-/Textual into MuteToggle
Minor clean up of winny's code and save mute state to world dictionary. I don't like Mute Sound being under File... but I don't know where else to put it.