Move the title display into a macro so that it can be globally overridden

Copy the idea behind 5b38c21a417d2e5e2b85aed8010c88af32420e24 so that we can replace depencies on overridden core tiddlers with mere macro overriding.

A typical use case is localised documentation, where we could add such a global timeline-title() macro whithout overriding the core timeline macro:
```
\define timeline-title()
<$view field="fr-title">
<$view field="title"/>
</$view>
\end
```