[processing] Refactor Fields algorithm: avoid unnecessary confirmation dialogs

Author: gacarrillor

python/plugins/processing/algs/qgis/ui/FieldsMappingPanel.py

@@ -318,6 +318,8 @@ def setDelegate(self, column_name, delegate):
             delegate)
 
     def setLayer(self, layer):
+        if self.model.layer() == layer:
+            return
         self.model.setLayer(layer)
         if layer is None:
             return

python/plugins/processing/tools/general.py

@@ -154,4 +154,5 @@ def execAlgorithmDialog(algOrName, parameters={}):
         canvas.setMapTool(prevMapTool)
 
     results = dlg.results()
+    dlg.close()
     return results