[processing] Avoid losing static, user-entered parameter values

in model child algorithms

Prior to this change if you edited an algorithm in a model and
tried to enter a preset string for certain parameter types, this
string would get silently discarded on closing the dialog. E.g.
with a dissolve algorithm it was not possible to have a fixed
field name within the model to dissolve by.

This was caused by WidgetWrapper.comboValue  returning the customData
for these manually entered values in the parameters combo box,
yet manually entered values never have custom data.

To work around this we only return the custom data if its
set for the selected item - otherwise we return the text unchanged.
In order to handle the "[not set]" options, a new static custom
data value of WidgetWrapper.NOT_SET_OPTION is added to that
comboValue can detect this and return the appropriate None value.

python/plugins/processing/gui/wrappers.py

@@ -140,6 +140,8 @@ def getExtendedLayerName(layer):
 class WidgetWrapper(QObject):
     widgetValueHasChanged = pyqtSignal(object)
 
+    NOT_SET_OPTION = '~~~~!!!!NOT SET!!!!~~~~~~~'
+
     def __init__(self, param, dialog, row=0, col=0, **kwargs):
         QObject.__init__(self)
         self.param = param
@@ -160,7 +162,12 @@ def comboValue(self, validator=None, combobox=None):
             if validator is not None and not validator(v):
                 raise InvalidParameterValue()
             return v
-        return combobox.currentData()
+        if combobox.currentData() == self.NOT_SET_OPTION:
+            return None
+        elif combobox.currentData() is not None:
+            return combobox.currentData()
+        else:
+            return combobox.currentText()
 
     def createWidget(self, **kwargs):
         pass
@@ -1303,7 +1310,7 @@ def createWidget(self):
             tables = self.dialog.getAvailableValuesOfType((QgsProcessingParameterVectorLayer, QgsProcessingParameterString),
                                                           (QgsProcessingOutputVectorLayer, QgsProcessingOutputMapLayer, QgsProcessingOutputFile, QgsProcessingOutputString))
             if self.param.flags() & QgsProcessingParameterDefinition.FlagOptional:
-                self.combo.addItem(self.NOT_SELECTED, None)
+                self.combo.addItem(self.NOT_SELECTED, self.NOT_SET_OPTION)
             for table in tables:
                 self.combo.addItem(self.dialog.resolveValueDescription(table), table)
 
@@ -1395,7 +1402,7 @@ def createWidget(self):
             fields = self.dialog.getAvailableValuesOfType([QgsProcessingParameterField, QgsProcessingParameterString],
                                                           [QgsProcessingOutputString])
             if self.param.flags() & QgsProcessingParameterDefinition.FlagOptional:
-                widget.addItem(self.NOT_SET, None)
+                widget.addItem(self.NOT_SET, self.NOT_SET_OPTION)
             for f in fields:
                 widget.addItem(self.dialog.resolveValueDescription(f), f)
             widget.setToolTip(
@@ -1508,7 +1515,7 @@ def createWidget(self):
             fields = self.dialog.getAvailableValuesOfType([QgsProcessingParameterBand, QgsProcessingParameterNumber],
                                                           [QgsProcessingOutputNumber])
             if self.param.flags() & QgsProcessingParameterDefinition.FlagOptional:
-                widget.addItem(self.NOT_SET, None)
+                widget.addItem(self.NOT_SET, self.NOT_SET_OPTION)
             for f in fields:
                 widget.addItem(self.dialog.resolveValueDescription(f), f)
             return widget