* feat: add block model to the visualisation

Add a Block Model button to the visualisation panel. It fills the model
bounding box with blocks, evaluates the stratigraphy at each block centre
on a background thread, and colours the blocks by the stratigraphic column.
The number of blocks along each axis is set in a dialog and defaults to
the model resolution.

* feat: update out-of-date viewer objects with a button

Model changes no longer rebuild the viewer objects automatically. The
objects built from the model are marked out of date (grey italic in the
object list), and a new button builds them again from the current model
on a background thread. Each object keeps its name, visibility and viewer
settings. Cross sections, block models and topography are also updated.

Also keep the source information when the properties panel adds an object
again, and save opacity changes.

* feat: filter viewer objects by value range or by unit

Add a Filter (Threshold) section to the object properties panel. For the
unit ids of a block model, cross section or coloured topography, it shows
a check box for each unit (with its name and colour); other arrays use a
value range, with an option to invert it. The full mesh stays stored, so
the filter can be changed or cleared, and it stays when the object is
updated from the model.

The objects coloured by stratigraphy now store the unit names and colours,
and the viewer gets replace_mesh_object to add an object again with the
same settings.

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