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

[roblourens]

To verify, just try everything listed at the top.

For

the cell at the top of the callstack, not the cell where the user clicked Run

I mean, have two cells, like

def foo():
    print('hello')

and a second cell that calls foo(). Set a breakpoint in the function definition, click run on the second cell, and make sure "Go To" goes to that first cell where the instruction pointer is.