disambiguation
resolved some of the single_file logic

- though it still feels too spread out.
- it’d be nice to not need the `single` kwarg in ImageExportSession
be more subdued with line-highlighting

- only highlight the current line in the frontmost window (and only if
the editor’s the first-responder)
completed the export() rewrite

- the export() command now uses the same export session objects as the
gui
- merged the Movie, ImageSequence, and PDF helpers into a single
ImageWriter context manager class (returned by export())
try to keep things sRGB