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?
This text box doesn't appear to get populated with the existing set of encouragements on display.  The best place to do this is by overriding `OnActivate`