Add `parent` property to `Tag` class

[audse]

This will allow tags to be hierarchical.

Tag.identifier should include the parent in the identifier, ex. (pseudo-code):

Tag tag_1("parent_name");
Tag tag_2("child_name");
tag_2.parent = tag_1;
assert(tag_2.get_identifier() == "parent_name/child_name")

[ODtian]

I this it should be something like ancestors array, therefore you can check whether it has a category of tag. Like you can query if it has any kind of buff, but you do not care the certain buff name.