...as it turns out, a first preliminary clip display should be working by now;
seemingly I was able to the tough theoretical problems last spring,
and was at the point of actually allocating display extension space
within the custom drawing area of the timeline.
Thus a simple placeholder widget based on a Gtk::Button should show up
at the right position, when sending a suitable diff message. The only
thing missing seems to be a first rough draft for the function
determineRequiredVerticalExtension()
The reason was: each further ViewRefHook added again the full offset.
Need to change the hierarchy and allow for this chained hooking already
starting from the base interface ViewHook onward (with trivial default impl)
...which erroneously assumed the list of timelines to be empty.
When sending a further population diff, this assumption is broken,
since the first diff resulted in adding a timeline element.
This misatke was detected by the new consistency check added with
9f3fe8a88
The population message is just made up, in order to create more interesting structures
in the UI and so to further the development of the timeline display.
For the actual structure I choose to mirror my example drawing in draw/UI-TimelineLayout-1.png
which is also used in the TiddlyWiki, on the #GuiTimelineView tiddler
https://lumiera.org/wiki/renderengine.html#GuiTimelineView
