This adds two new APIs - `drag_pair_forwards` and its companion
`drag_pair_backwards` which allow dragging 'pairs' of elements within a
form.

An easy example is the key-value pairs within a map which typically are
meant to stay together when reordering.

This can, however, be used for any form. This means if you have some
vector containing logical pairs (because of the actual semantics of your
code) you can use a dedicated keybinding to drag them around together.

This change also introduces a new config option at:

`dragging.auto_drag_pairs = true|false`

which will alter the behaviour of the existing `drag_element_forwards`
and `drag_element_backwards` APIs in order to try infer whether they are
contained within a node that is made up of pairs.

For example if this setting is `true` and a `drag_element_forwards` is
used on the keys of a map then they will be dragged pairwise.

This new config option defaults to `true` under the assumption that this
is generally the desired behaviour.