select first record only if nothing is selected

Otherwise the first record is always appended to the selection when a focus
event happens

review779003

tryton/gui/window/view_form/view/list_gtk/parser.py

@@ -298,7 +298,7 @@ class ParserTree(ParserInterface):
 
     def set_selection(self, treeview, direction):
         selection = treeview.get_selection()
-        if len(treeview.get_model()):
+        if len(treeview.get_model()) and not selection.count_selected_rows():
             selection.select_path(0)
         return False