When duplicating a layer copy layer tree layer's custom properties [fixes #60884]

[Alex-Kent]

Description

When using Duplicate Layer in the Layers panel only the custom properties for the QgsMapLayer were being copied with any custom properties for the associated QgsLayerTreeLayer being omitted.

This PR fixes that issue by copying the source layer tree layer's custom properties to the new layer tree layer.

I've tested the code and it behaves as expected. Test cases included those described in the bug report as well as with the Layer Color Plugin (which is affected by this bug).

Backporting this fix to the LTR releases would be nice.

Fixes #60884

[signedav]

Nice catch. Code looks good. Did you check if any custom property used from QGIS itself might make no sense to be duplicated? Just to be sure.

[Alex-Kent]

Did you check if any custom property used from QGIS itself might make no sense to be duplicated?

There is a list of some custom properties used by QGIS with QgsLayerTreeNode (the base class for both QgsLayerTreeLayer and QgsLayerTreeGroup) in the class' documentation; there does not appear to be a similar list for QgsMapLayer. I can't say whether the list on that page is definitive/complete. Regarding your query, all of the custom properties listed in the class documentation seem sensible to duplicate.