Use combined Audience in Audience Library (Not just "Activity-Only")

[#1]

Currently the only way to create a combined audience is from inside of an activity. Additionally when you create a combined audience from the activity in question it is only an "Activity-Only Audience". It does not become available in the "Audience Library" to be used in other activities.

The feature being requested is that we should be able to:

a.) Build a combined audience.

b.) Check a box to have this audience show up in the Target Audience List so that we can use it in any activity we choose to.