LP#1788417: boolean fields should say 'Yes'/'No' instead of true/false

boolean fields on egGrid use tt2-translated strings for "Yes" and "No"
instead of letting the browser translate the values 'true' and 'false'
(in accordance with the prophecy of XUL)

Signed-off-by: a. bellenir <ab@grpl.org>
Signed-off-by: Jason Stephenson <jason@sigio.com>
Signed-off-by: Chris Sharp <csharp@georgialibraries.org>