Add weight to links spec

Add weight to links spec

This allows a weight to be set & for it to alter the order.

It is a bit hard to know whether to add weights to existing ones as
it would be a lot to do all of them - but I did hear a specific request
(can't recall who from) to have Delete last to reduce mis-clicks
and with the View & Edit declared in the same place
I felt I should add them with low values for some sort of consistency

This would mess with people who have implemented weight in a hook - but
probably any change is low impact & an easy-ish fix

Note that I will update docs once agreed