result layers from algorithm run from the EnMAP-Box processing panel shouldn't be opened in the QGIS Layers panel

[Andreas Janz]

As discussed with @Benjamin Jakimow in #937.

How to reproduce:

• open enmap_berlin in a MapView
• run the GDAL Translate algo from the EnMAP-Box Processing Toolbox to create a copy
• the result layer will show up in the QGIS Layers panel, which is not good

The result layer should only show up in the EnMAP-Box Data Sources panel, instead of also messing up the non-related QGIS Project layers list.

[Benjamin Jakimow]

requires improvements to QGIS API