Refactor scroller so that the scrollIntoView() method is a bit more usable

Passing an event instead of a DOM node was silly.
Special treatment so that text reference to title field works for missing tiddlers

The title field is the one field that we know for missing tiddlers.

This is needed in a subsequent commit of viewswitcher.
Add a hack to switch slowmo animation mode

Just type `$tw.slowmo()` in the developer console to engage slowmo mode.
Add back the zoomin view

An alternative visualisation for the story list. It just shows the
currently selected tiddler, with smooth animations between tiddlers.

Not entirely finished yet; for instance, if you close the current
tiddler you get a blank screen until you select another tiddler.
Plumbing to allow the current view to be switched