Improve awareness of debug location in notebooks

[roblourens]

It's easy to lose track of the debugged cell in notebooks, especially when you've stepped into a cell which is different from the cell that is "running". Some improvements we could make:

• The "Go To" button should go to the cell at the top of the callstack, not the cell where the user clicked Run
• We should show overview ruler decorations for the top stackframe, as in the editor
• And overview ruler decorations for the currently running cell
• And for breakpoints (behind a setting, debug.showBreakpointsInOverviewRuler)

cc @kieferrm