This eliminates the need to track and iterate through a separate array of `Accessible`s in `ScreenReader`.
The previous performance issues are resolved by ensuring that any nodes processed aren't children of the `ScreenReader` node. I don't know of another way to avoid triggering signals on a child node than checking ancestry and failing fast.
Here I add a `ScreenReader` `Node` which, when added to the `SceneTree`, should read the contents of any supported `Control`. I also do a bit of cleanup on filenames, as well as breaking the editor plugin support into its own class.
