Split adding/creating views into an object

This object gets instantiated when the package is activated, and
destroyed when the package is deactivated
Rename PreviewTabsView to PreviewTabsPaneView
Rename PanesManager tests
Don't inherit from View in PreviewTabsPaneView
Rename PaneView to PaneController

Since it no longer inherits from View.
Inline callbacks for PreviewTabsPreview

They were all `keep()`, so just inline them.
Update class top-level comments
Inline unsubscribe function in PaneController
Merge branch 're-organize'