Default the third pane for "Assign Abbreviation" to the matched string.

I've always thought that a blank third pane is not very useful.

Often I want to fix the ranking of an object after I've arrowed down to it. This diff makes it easy to fix such a problem:
- Run "Assign Abbreviation" on the item, which will default to using what you searched.
- Invoke QS again, and run the intended action.

![screenshot 2013-07-31 16 45 30](https://f.cloud.github.com/assets/248078/890698/56ad8502-fa3b-11e2-8baf-179695c2eeae.png)

`matchedString` appears to be the proper accessor exposed by `QSSearchObjectView`, although I don't know if it might have issues with e.g. special keyboards/characters.
However, this does work if you use the action with a dismissed-and-re-invoked QS direct object selected via text. (A hackier approach using `partialString` didn't work.)

Note that selections sent to Quicksilver don't have a `matchedString`, so there's no sensible suggestion (although you *could* make an argument for either the entire string version of the direct object, or for a string of the first letter of every alphabetic substring).