mark lines that have been edited since last save

[d1manson]

Currently there's the asterisk and widget text to tell you when something in the document has changed since the last save, and there are the save-agnostic change markers in the gutter, however it might be nice to combine the two so as to make it clear which lines have been modified since the last save.

Note that this has to be integrated into the undo/redo stack (as is currently done for the asterisk and widget text), i.e. if you make a change and then undo the change it needs to be treated as though no change was ever made.

Originally requested on Google+.