Add property page for encouragements

This adds the simplest possible property page for customizing encouragements.

The only problem so far is that you can't hit `Enter` in the text box to add another encouragement. So if you need to add multiple, you have to cut and paste.

The "OK" button seems to steal the Enter keypress. I tried to handle it in the KeyDown event, but I think the OK button receives it first.

@jaredpar any thoughts on what we can do here?
If the user is feeling a bit evil they can clear out all encouragements.  This will cause this code to throw an exception because it will end up indexing into an empty collection.