Function getLayerTreeElementsById

getLayerTreeElementsById(): {
[key in JId]: JLayerTreeElement
}
JMap.Layer.getLayerTreeElementsById

Returns a map (= a javascript object) where :
- the key is the tree element id
- the value is the tree element
A tree element is a layer group or a layer.

A layer group contains layer(s) and/or other layer group(s), and has a negative id.

A layer has a positive id.

If no project is loaded, returns en empty object.
Returns {
[key in JId]: JLayerTreeElement
}
Example
```
// returns a map of layer elements defined by layer id
JMap.Layer.getLayerTreeElementsById()
```