* Introduce QgsLayerTreeCustomNode to handle items in QgsLayerTree which are not layers nor layer groups (e.g., a custom profile source from a web service to the Elevation Profile layer tree); make QgsAbstractProfileSource and its generator classes get an id in the constructor, so that subclasses don't need to deal with source ids (source ids from the generators are already in use, whereas source id from source subclasses will be used to link sources to QgsLayerTreeCustomNodes, and will be used to retrieve sources from source registry)
* [elevation] Add/remove nodes to Elevation's layer tree when registering/unregistering profile sources via QgsProfileSourceRegistry
* [elevation] Add updateCanvasSources() to Elevation Profile widget, so that after changes in the elevation layer tree, the canvas gets an updated list of profile sources, taking profile sources (i.e., from custom nodes) into account. Add QgsProfileSourceRegistry::findSourceById() for getting profile sources by id from the registry.
* [elevation] Allow for setting a display name for custom profile sources (to be displayed as layer tree node name)
* [elevation] Refactor profile source handling in Layout Elevation Profile widget, so that after changes in the elevation layer tree, the canvas gets an updated list of profile sources, taking profile sources (i.e., from custom nodes) into account. Adjust QgsLayerTreeGroup to be able to find layers and custom nodes, as well as order layer and custom nodes inside a group (only direct children).
* [elevation] Profile Source Registry improvements:
+ Avoid registering a source if its id is already registered,
+ New method to unregister a source by id, deprecating the unregister by source object.
* [tests] Add tests for QgsLayerTreeCustomNode
* [tests] add tests for legacy Elevation Profile sources and for legacy Layout Profile items:
+ make sure we keep the API untouched and mark some stuff as legacy (e.g., pure virtual methods in QgsAbstractProfileSource need to have a default implementation and making them pure virtual is postponed until QGIS 5.0),
+ add insertCustomNode(index, node) method to QgsLayerTreeGroup, so that we can set up the custom node before adding it to the layer tree, because if we add it with no custom properties set (e.g., for the elevation profile custom node), it will trigger plot updates but it won't be found due to a still missing custom property.
* [elevation] Make sure legacy layer ordering in canvas, layout item profile, and layer tree are kept (i.e., keep API untouched). Make profile source ordering works in the most expected way: canvas, layout item profile, and layer tree are in the same order, and sources are only reversed right before passing them to renderers. Add unit tests.
* [tests] Custom node for custom Elevation profiles: make sure layout item sources are stored in and read from the project
* Address review on custom nodes for elevation profile's tree view
