Bookmarks to lines of code within a notebook,

Hi,
This is not a dupe of 545, but is about being able to quickly browse within a notebook.
I'd like to able to click in the margin and have some sort of marker appear.

Within a notebook there would be a way to see the lists of marks.
This is not about seeing bookmarks to other notebooks, just being able to quickly get back to places in a large notebook.

This would be especially useful when using an embedded shell in the debugger and scrolling up and down the notebook.

No files, tests, or entry points are named. Start by examining the notebook interface and the existing bookmark-related work referenced by issue 545; done would mean a defined way to mark positions within a notebook and view or return to those marks, including while using the embedded debugger shell.

Written by the indexing model from the issue text.