Refactor idea: Define stages, unify the interfaces

[xiaomi7732]

Shall we unify all PreFilter, Parser, Beautifiers and Filters into:

IFilter
{
    enum Stage { get; }    // PreFiltering, Parsing, Beautifying, Filtering...
    int Priority { get; }     // Priority within the stage;
}

• The engine doesn't need to predefine the stages any more.
• It picks up all the registered filters, sort them by Stage then by Priority, and runs them though totally.