subform list removal does not trigger model re-render

reproduce:

• insert 3 contributors
• name them 1, 2 and 3
• remove contributor 2
• the yaml file is correct: contributors 1 and 3 are still here and 2 has been removed
• the form is incorrect: v-model did not update on the field and it shows contributors 1 and 2 instead of 1 and 3

reason: not sure why, but the removal of an item in the array does not trigger an update of the sub-component's v-model.

fix: for the same issue on list of simple fields, we added :key="index + dummy" and increment dummy after removing the item in the array, leading to issue #9 (closed). We tried to do it nonetheless, but it creates an infinite loop of updating.