add move_shortcut / change_shortcut method

[ivanov]

while replying to a question on the mailing list, it occurred to me that we currently make it too difficult to simply remap functionality of a default keyboard shortcut to a different key. The API for this could be something like move_shortcut('ctrl-k', 'shift-k') to make shift-k do what ctrl-k used to do.

[Carreau]

IMHO, shortcut should be bound to strings, and strings to actions.

Also remove shortcut could return the currently set value so that it is easy to move around.