VisualEditor.php: Set default preferences values

This not being here caused and causes various unexpected
scenarios to evolve around preferences being falsy (undefined)
or set to 0, and inability to distinguish between a user having
not set the preference, the preference not existing due to cache
or the preference being disabled explictly by the user.

Change-Id: Ie50b63ba5064e85d26dad8b622554bbe809c2634