rewrite the custom raster renderer to be applications

[Andreas Janz]

Add context entries using Benjamins code snippet:

‌

def onCreateContextMenu(menu: QMenu):
    view = QApplication.instance().sender()
    if isinstance(view, DockTreeView):
        node = view.currentNode()
        print('Add context menu actions')
        # consider node / selection context
        a = menu.addAction('MyAction')

emb = EnMAPBox()
emb.dockTreeView().sigPopulateContextMenu.connect(onCreateContextMenu)

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