markAvailable - marks all the available magnets or elements when a link is dragged (being reconnected). Default is false. This gives a hint to the user to what other elements/ports this link can be connected. What magnets/cells are available is determined by the validateConnection function. Internally, available magnets (SVG elements) are given the 'available-magnet' class name and all the available cells the 'available-cell' class name. This allows you to change the styling of the highlight effect.