Fix multi edit relation reference

[nirvn]

Description

This PR fixes an issue when the attribute form mode is set to multi editing whereas relation referencing editor widget value is wrongly marked as "edited" upon opening the attribute form (i.e. prior to actually editing the widget value).

Note the wrong "value changed" red button in the feature form upon opening the form

It's a bit of a rabbit hole to follow here, but basically what happens is that the relation referencing editor widget sets the foreign keys value, which is first consumed as a non-found feature as the feature picker model gathers features. Upon having successfully gathered the feature, the model reports that the feature is found. Until now, it was using the feature changed signal, which bubbled back up to the relation referencing editor widget as a "foreign key changed" even though it clearly isn't.

The way around this without a significant refactoring is to add another signal that identifies a feature found state change (vs. using feature changed for both an actual feature values change and a feature values found).

[github-actions]

🪟 Windows builds ready!

Windows builds of this PR are available for testing here. Debug symbols for this build are available here.

(Built from commit 21fa5cd)

[github-actions]

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