Update JSONListModel.qml

[think-free]

Intelligent update of the elements (without destroying everything each time the model is updated). You have to specify which is the unique identifier of the element : 'propId' of the JSONListModel

[tanghus]

Hi,

I'm about to try out this long-living model :dancer: :smile:

Should this PR be merged first?

[kromain]

heh, I even forgot there was still an open PR! I haven't used it in years but I'll try to take a look asap. Glad to hear this can still be of use to you and others. Thanks everyone for the contributions

[tanghus]

Sometimes old sins sneak up on you ;)