Fix Xcode UTF-8 vs. Unicode warning
Refactored ApplicationController -setIcon to -setNotificationIcon and -setDefaultIcon, with Debug menu.
Pass nil to setApplicationIconImage instead of defaultIcon, so user-set icon is preserved.

This preserves any custom icon assigned to MacIrssi.app using Finder.
Passing defaultIcon which came from [NSApp applicationIconImage] forces
the icon included in the bundle, overriding the custom icon.

As a result of this change, neither defaultIcon nor currentIcon need to
be tracked.

NSApplication setApplicationIconImage docs say:
  To restore your application’s original icon, you pass nil to this method.
Update irrsi to v0.8.16-rc1