Nyall Dawson bbd8a5c902 Add option not to overwrite existing property in
QgsAuxiliaryLayer::createProperty(*)

Instead, if a property already exists it will be upgraded to
an expression based property of the form:

    coalesce("new aux field", 'existing' || 'property' || 'expression')

(i.e. allow per-feature value overrides from the auxiliary field,
but by default fallback to the existing property definition)

Refs #44475
2021-09-28 13:25:54 +10:00
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