feat: add vertices instead of deleting all old ones

Author: merydian

ORStools/gui/ORStoolsDialog.py

@@ -793,8 +793,6 @@ def load_vertices_from_layer(self, testing: str = "") -> None:
         if result == QMessageBox.StandardButton.Ok:
             layer = box.selectedLayer()
             try:
-                self.routing_fromline_list.clear()
-
                 for id, feat in enumerate(layer.getFeatures()):
                     geom = feat.geometry()
                     if not geom: