Add interaction between the QgsProcessingParameterMultipleLayers widget and the layertree

[roya0045]

Feature description

I swear I saw a PR or a FR at some point about dragging and dropping layers into that widget but I could never get it to work.

This request is to have a way to take into account selected layers in the layer tree in the widget.

I can see three way of doing this:

1. Have layer selected in the main layertree already selected in the widget (similar to what is done with single layers)
2. Add a button to select the layers that are 'active'/selected in the main layertree
3. Add the option to drag and drop layers in the widget (the line of the list widget) as a way to select them.

Additional context

If this is doable/merged or already present delete this FR, I couldn't find anything relevant when I searched.

[DelazJ]

@roya0045 #30179?

[roya0045]

@roya0045 #30179?

Maybe that's what I recall but I'm sure it was with the multiple selection widget and not the layer one... Though that PR might be a good base to improve it. Thanks!