Add STTextFinderBarContainer to handle text finder bar

Move responsibility for managing the text finder bar from STTextView to a
new STTextFinderBarContainer class that implements NSTextFinderBarContainer.
This allows properly positioning the gutter view below the find bar when it
is shown/hidden.

Update STTextView to:
- Create the text finder client, bar container and finder in init
- Connect the text finder client and bar container
- Remove code to set up the text finder when moving to a window
- Scope some enclosing scroll view accesses