Pane/PaneView API changes

This combines the Pane and PaneView APIs and removes some underused methods from the API.

One thing we might want to consider, move the split methods from Pane to
Workspace. I think conceptually it makes sense to split the Workspace rather
than a Pane. 
Even if we add `activateItem` on line https://github.com/atom/api/pull/2/files#diff-149992b996073602a05797d7a977ba4aR407