Joins in save/load QML Styles

[signedav]

Feature description

We do support to export / import lots of the layer properties to / from QML style files (saveNamedStyle / loadNamedStyle), but joins are not yet supported.

The layer joins to other layers

And has the fields available then in the attribute form

The complexity on the integration is, that a join always depends on another layer. And a QML is concerning only one layer. Still the Relations are supported. This might be a similar implementation.

Additional context

No response

[signedav]

@elpaso I remember that you did (at least parts of) the weak relations. I guess this goes into the same topic. My question to you: Have you checked already some parts of it? Do you think the implementation should go into the same direction, or did you encounter some limitations?