make qt HistoryList.on_update() faster by caching icons

gui/qt/util.py

@@ -393,6 +393,8 @@ class MyTreeWidget(QTreeWidget):
         self.addChild = self.addTopLevelItem
         self.insertChild = self.insertTopLevelItem
 
+        self.icon_cache = IconCache()
+
         # Control which columns are editable
         self.editor = None
         self.pending_update = False
@@ -779,6 +781,17 @@ class SortableTreeWidgetItem(QTreeWidgetItem):
             return self.text(column) < other.text(column)
 
 
+class IconCache:
+
+    def __init__(self):
+        self.__cache = {}
+
+    def get(self, file_name):
+        if file_name not in self.__cache:
+            self.__cache[file_name] = QIcon(file_name)
+        return self.__cache[file_name]
+
+
 if __name__ == "__main__":
     app = QApplication([])
     t = WaitingDialog(None, 'testing ...', lambda: [time.sleep(1)], lambda x: QMessageBox.information(None, 'done', "done"))