VT100: Add a "Notification" extension to allow for remote sending of notifications

The syntax is similar to the File extension: a semicolon-delimited string of
arguments. Currently, two arguments are supported: title, and description.

NB: At this stage, this is mainly an RFC/work in progress. This is the second time I've ever used objc in my life, so, mandatory:

![i have no idea what i'm doing](https://t1.gstatic.com/images?q=tbn:ANd9GcQ5nkQp1NO10U5rEong1pZ2wYlzBo-DQBvVNzxLICFaBdTxnRGU)

I've attempted to mark things that I think need fixing for real with TODO. I'm happy to work on them, but probably need some direction/help ;)
This file shouldn't know about iTermGrowlDelegate. Its only interface is through its delegate methods (e.g., terminalPostGrowlNotification:), which are implemented by VT100Screen. You should add a new delegate method to VT100TerminalDelegate.h along the lines of terminalPostGrowlNotificationWithTitle:(NSString *)title description:(NSString *)description. Implement it in VT100Screen; it should be very similar to the existing terminalPostGrowlNotification:. In fact, since it's just a 2-arg version of terminalPostGrowlNotification:, make terminalPostGrowlNotification: call this new method with @"Alert" as the title.