Make tags.none: filter work again (#3877)

* Don't remove the legacy 'tags' property

* Simulate the `tags` DOM property

This refers to the deprecated "tags" property of tasks. This property
usually still exists, for compatibility with older versions, but should
not be relied on anymore.

src/DOM.cpp

@@ -286,6 +286,13 @@ bool getDOM(const std::string& name, const Task* task, Variant& value) {
       return true;
     }
 
+    // The "tags" property is deprecated, but it is documented as part of the DOM, so simulate it.
+    if (size == 1 && canonical == "tags") {
+      auto tags = ref->getTags();
+      value = Variant(join(",", tags));
+      return true;
+    }
+
     Column* column = Context::getContext().columns[canonical];
 
     if (size == 1 && column) {

src/commands/CmdEdit.cpp

@@ -317,7 +317,6 @@ void CmdEdit::parseTask(Task& task, const std::string& after, const std::string&
 
   // tags
   value = findValue(after, "\n  Tags:");
-  task.remove("tags");
   task.setTags(split(value, ' '));
 
   // description.