Processing Algorithms: Spatial Extent Widget does not show layers

[jakimowb]

The SpatialExtent widget does not list the layers which are opened in the EnMAP-Box.
All of it's context menu entries relate to the main QGIS map canvas but not that used in the EnMAP-Box.

• Calculate from Layer -> should allow to select EnMAP-Box layers
• Calculate from Layout Map
• Calculate from Bookmark
• Use current map canvas extent
• Draw on Map Canvas -> should allow to draw on EnMAP-Box canvas

Workaround

ensure that current layers are temporarelliy added to QgsProject.instance()

Reason

The QgsProcessingExtentParameterDefinitionWidget does not consider the context / widgetContext, e.g. to use a QgsProject different to QgsProject.instance().

How to reproduce

1. Open EnMAP-Box
2. Project -> Add Example data
3. Data Sources, Context menu "enmap_potsdam" -> open "Translate Raster Layer"
4. Open Advanced Parameters and try to set the Spatial Extent