[elevation Profile] Correcly add a layer generated by a processing

[ptitjano]

Issue

This is somewhat similar to #59809

1. Create a new project
2. Open a new elevation profile
3. Launch any processing to create a new Layer
4. The new layer is not added to the elevation profile widget

Solution

This is because QgsLayerTreeRegistryBridge listens to the QgsProject::legendLayersAdded() signal in order to update the elevation profile tree view. However this signal is not triggered by the current logic in the Postprocessing.py.
This PR changes the behavior of Postprocessing.py. by triggering the QgsProject::legendLayersAdded() signal and removing the duplicated node in the tree view.

This seems like a bad solution but I'm not sure what the correct fix. Should QgsLayerTreeRegistryBridge listen to an other signal? Which one? Should Postprocessing.py be modified?
Any thoughts?

cc @Djedouas

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[nyalldawson]

@ptitjano

This seems like a bad solution but I'm not sure what the correct fix.

Indeed 😆 (not to mention it's only a partial fix -- if the layers are being placed in a specific group then the fix won't work either).

It makes me nervous seeing a lot of changes to Postprocessing.py. This is quite a fragile file (with no unit tests at all!), and we usually get regressions whenever it's modified. (And unfortunately we can't port it to c++ for compile-time safety, because it relies on RenderingStyles and ProcessingConfig, which are currently python only).

Should QgsLayerTreeRegistryBridge listen to an other signal? Which one? Should Postprocessing.py be modified?

Maybe we could work around this by moving the logic postprocessing which adds the layers to the tree directly to QgsLayerTreeRegistryBridge. I.e this loop: https://github.com/qgis/QGIS/blob/master/python/plugins/processing/gui/Postprocessing.py#L280-L296

[ptitjano]

Maybe we could work around this by moving the logic postprocessing which adds the layers to the tree directly to QgsLayerTreeRegistryBridge. I.e this loop: https://github.com/qgis/QGIS/blob/master/python/plugins/processing/gui/Postprocessing.py#L280-L296

Could you explain a little bit how this could be achieved please? I don't know how QgsLayerTreeRegistryBridge works and I can't find any example.

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