Issue

Partially resolves #12.

Details

Adds behaviours to trap navigation to a dialog when it is modal. I.e.:

Assistive technologies MAY limit navigation to the modal element's contents. If focus moves to an element outside the modal element, assistive technologies SHOULD NOT limit navigation to the modal element.

Does not add the behaviour to move the virtual cursor to the modal. I.e:

When a modal element is displayed, assistive technologies SHOULD navigate to the element unless focus has explicitly been set elsewhere

TBH, not sure how feasible this is due to it's abiguity... what does "is displayed" mean?

CheckList

• Has been tested (where required).