Warning before discarding information longer than shapefile field length

[qgib]

Author Name: A E (A E)
Original Redmine Issue: 16929

Redmine category:attribute_table

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Steps to reproduce:

0: needs a shapefile with a value relation field (e.g. a linked species list) using multiple choices option

1. set a shapefile layer in editing mode,
2. open attribute table,
3. select a feature,
4. open form view,
5. go to a value relation field of the attribute form,
6. select many entries, summing up to a total length longer than the field length
7. end editing mode and save changes

Now, check the entry in the attibute table field. The entries will have been cut off on reaching the maximum field length of the attribute.

Suggested behaviour: Prompt a warning on selection of any surplus attribute in the value relations field, or, if not possible, before saving the shapefile (with option to continue in editng mode w/o saving).

(See also Ticket #24827)

[alexbruy]

See also #15139, #20102