CRM-4198 Activity Types should indicate whether they are online or offline

It would be useful if Activity Types could be classified as online or offline. My particular use case is that I am exposing the activity types (via the API) to our CMS for creating online action forms. It's useful to assign an action form to an activity type (like "Petition", "Email", etc.) but the activity types like "Interview" are not useful to display in that context as options because they're offline-only activities.

I'm not sure if it would be better to implement a generic categorization system for activity types or just an online / offline toggle. Either one would work for my needs.