Selectors should inherit from Selector, not ObjectSelector

[jbednar]

ObjectSelector is old, deprecated API, replaced with Selector. However, FileSelector, ListSelector, and MultiFileSelector still inherit from ObjectSelector, making it difficult to remove and making those classes confusing to work with. Swapping ObjectSelector and Selector in the class hierarchy shouldn't affect anything not testing those exact classes, and we should do that soon to avoid future confusion.