Added keyboard shortcuts à la WinMerge.
Alt + Up or Down arrow : go to previous or next difference
Alt + Left or Right arrow : merge difference using right or left content

Shift, Ctr, Alt or Meta keys can be used indifferntly (to cope with browser's idiosyncratics).
Event handler is now attached to text editor. This is clearly to review (does not capture all page events and too intrusive).

New state "current_diff" has been added.

Need to scroll the editor to center it on currently active state (scrollToChange function to add).