Add a Rules condition for "Relationship is one-way"

I'm building an activity stream module that relies entirely on Rules to set activity messages (long story). I'm trying to provide some default Rules, which means I can't rely on the assumption that any specific relationship types will exist; however, I want to display different messages depending on whether a relationship type is one-way or two-way. For example, I want to display a message on users' profiles when they make a new friend (two-way) but not when someone follows them (one-way).

The attached patch adds a condition to Rules that checks whether the relevant relationship is one-way.