Preserve user-specified dock icon after activity notification.

It's possible to use Finder to Get Info for MacIrssi.app and replace the icon with [something awesome](http://potential.deviantart.com/art/MacIrssi-Alternative-Icon-147762197).

However, `[NSApp applicationIconImage]` still returns the path to the bundled icon, so when the activity notification is being cleared and MacIrssi calls `[appController setIcon:defaultIcon]`, the user-specified icon is not used.

Passing `nil` instead of `defaultIcon` does the trick. [NSApplication setApplicationIconImage documentation](http://developer.apple.com/library/mac/#documentation/Cocoa/Reference/ApplicationKit/Classes/NSApplication_Class/Reference/Reference.html) agrees:

> To restore your application’s original icon, you pass nil to this method.

I've also added Debug menu items to test switching between the icons. To do so, I refactored `AppController -setIcon:` out into `-setNotificationIcon` and `-setDefaultIcon`.

It no longer seemed necessary to track `currentIcon` and `defaultIcon`, so I've removed these. This means there's no longer a guard around `setApplicationIconImage` checking if the icon is indeed changing, but that seemed like an unnecessary optimization.. unless there's something I don't know?

Cheers!
Paul