Editor: warning when using arrow keys after changes

[rsimon]

When the user makes a change to the annotation, it is only saved on "OK". That means if left/right arrow key is used to skip to prev/next annotation after making a change, that change will be discarded. Correct behavior, but might still be a mishap by the user (and happen repeatedly without the user noticing).

Add a popup prompt that warns & provides options to save, cancel and "don't show this anymore".