[processing] add icons to SAGA algorithms (fix #18211)

(cherry picked from commit 4777abb)

Author: alexbruy

python/plugins/processing/algs/saga/SagaAlgorithm.py

@@ -30,6 +30,7 @@
 import importlib
 from copy import deepcopy
 from qgis.core import (Qgis,
+                       QgsApplication,
                        QgsProcessingUtils,
                        QgsProcessingException,
                        QgsMessageLog,
@@ -101,6 +102,12 @@ def groupId(self):
     def shortHelpString(self):
         return shortHelp.get(self.id(), None)
 
+    def icon(self):
+        return QgsApplication.getThemeIcon("/providerSaga.svg")
+
+    def svgIconPath(self):
+        return QgsApplication.iconPath("providerSaga.svg")
+
     def flags(self):
         # TODO - maybe it's safe to background thread this?
         return super().flags() | QgsProcessingAlgorithm.FlagNoThreading