Also give a better default if `alt` is not specified, using
the stringified caption rather than the filename.
Partially addresses #11140.
Add failing test case for Markdown writer issue in #11140.
Fixes#7473
Previously, `figclass`es weren't being extracted correctly.
`figclass` and `align` annotations are now implemented following the rst docs: these options apply to the figure while other options are passed onto the underlying image.