`TabData` recorded the type of the timeline the user had added to a tab.
`TimelineKind` is another type that records general information about
configured timelines, with identical properties.
There's no need for both, so remove `TabData` and use `TimelineKind` in
its place.
`TimelineKind` is itself mis-named; it's not just the timeline's kind
but also holds data necessary to display that timeline (e.g., the list
ID if it's a `.UserList`, or the hashtags if it's a `.Hashtags`) so
rename to `Timeline` to better reflect its usage. Move it to a new
`core.model` module.
Nik Clayton