Add Std_SelBack and Std_SelForward to the Std_TreeViewActions group where they belong.

This also solve the fact that these commands were not in the menu which are supposed to be exaustive.

src/Gui/CommandView.cpp

@@ -3880,6 +3880,11 @@ public:
 
         addCommand(new StdTreeDrag(),!cmds.empty());
         addCommand(new StdTreeSelection(),!cmds.empty());
+
+        addCommand();
+
+        addCommand(new StdCmdSelBack());
+        addCommand(new StdCmdSelForward());
     }
     const char* className() const override {return "StdCmdTreeViewActions";}
 };