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?
The `encouragements` collection is never updated here except on start up.  Doesn't this mean that encouragements won't update until after you restart Visual Studio?