"Run all modified" button / keyboard shortcut

[kolia]

"Run all modified" would be the fastest way to get the code in sync with the outputs, without having to re-run all cells.

(If keyboard shortcuts will be customizable, the choice of shortcut doesn't matter, but I suspect I would use the "run all modified" action all the time, instead of running individual cells, and only use "run all" if it looks like execution is in an inconsistent state.)

[fonsp]

Yep! I think that this should actually replace the "run all" button - or the button can be "synchronise" if there are changed cells, and "restart all" otherwise.

[kolia]

Per #89, it would be good to make sure that "Run all modified" considers global variables that have been updated asynchronously as having been modified. nevermind, see #4 and discussion in #89

[fonsp]

and