shorten ID-Label text to fit into size constraints

[Ichthyostega]

As Lumiera user,
I want long names of clips in the Timeline to be shortened (instead of completely hidden),
to make the Clips fit into a given temporal extension

technical spec

The basic framework for adapting to size constrained layout is already in place and integrated into ElementBoxWidget; however, the implementation just hides the name-ID completely, in case it can not be displayed within a size constrained widget. Obviously it would be far more adequate, just to shorten the names.

• find out what needs to be done to get an immediate feedback from GTK about the resulting screen extension after shortening a text
• find a solution how to retrieve the full text, once there is again enough space for display.
  • Beware: Clip display in a large Timeline creates a significant leverage
  • it might not be feasible just to duplicate that string for each and every widget
  • also it might be dangerous to heap-allocate an additional buffer when shortening Clip labels
  • further investigation is thus indicated, see #1240
  • possibly we need to do an asynchronous round-trip to the session, where the full name-ID is always retained
• moreover, find out how a given number of characters translates into pixels
• store this heuristic finding in the Advice system and use it for further layout calculations
• finally integrate this elaborate scheme at the marked point in element-box-widget.cpp

[Ichthyostega <prg@…>]

In ea53ef34/Lumiera:

ElementBox: coping with size constraints (closes #1238)

devise a more fine grained algorithm for adapting the display of IDLabel
to a situation with size constrained layout, e.g. for a time calibrated canvas.
We still do not implement the shortening of ID labels (see #1242),
since doing so would be surprisingly expensive. But at least we
do proceed in several steps now

• first attempt to reduce the name-ID (for now: hide it)
• if this doesn't suffice, also hide the menu
• and as a last resort, hide the icon and thus make the IDLabel empty

[Undercover Agent]

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