Copy the text editor string (when creating new objects) don't reference it

For an example of the bug see https://github.com/quicksilver/Quicksilver/pull/1532#issuecomment-22485744

This is the weirdest bug I've ever worked on, and it intrigues me how:

a) nobody has ever picked up this bug before (it's been around since for ever I think)
b) it hasn't affected QS that much in the past

But anyway, if you see the teeny tiny disclaimer in [the docs](https://developer.apple.com/library/mac/documentation/Cocoa/Reference/ApplicationKit/Classes/NSText_Class/Reference/Reference.html#//apple_ref/occ/instm/NSText/string) you'll see that 'for performance reasons' the *same* NSString object is returned every time. We want to store it (in our new QSObject), so we have to make a copy of it. Turns out it seems the NSNotification that I've changed does the same thing

Based against release branch