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. 

It might also allow us to simplify Workspace::open by adjusting how split works

```
Workspace::splitRight([items or uri]) # returns Pane
```
 
Then we could remove the `split` option from Workspace::open. So instead of:

```
atom.workspace.open(uri, split: 'right')
```

you would call

```
pane = atom.workspace.splitRight(uri)
pane.activate()
```

I think we'll need this for completeness. Can I only operate on the active item?