Fix adding a relation within multiple edition

[Djedouas]

Description

Context

A 1-N relation between a layer A ("projet" in example) and layer B ("document" in example) where one feature in layer A can have multiple linked features in layer B.

Bug

1. Correctly configure the relation between the layers and the relational widget in layer A
2. Select multiple features from layer A
3. Clic on the multiple edition tool
4. Add a new relational feature (in layer B)

5. Fill-in the attributes of the new feature (layer B)

6. EXPECTED: the new feature should be copied for every feature selected in layer A
   BUG: the attributes are not copied

Fix

The first feature is correctly added but the other features were added too early, before the user had filled-in the attributes.

This was due to the modality of the attribute form when adding the layer B feature.

With this form now modal, QGIS waits to have all the attributes for all the features to be filled-in.