feat(dia.Paper): custom events on link label

[MartinKanera]

Description

In case a custom event is bound to the label of the link propagate it, so the appropriate listeners can react to it accordingly.

Related to the changes from #2110.

Motivation and Context

Custom events that were declared on the label of a link like this:

const link = new joint.shapes.standard.Link({
    source: { x: 50, y: 50 },
    target: { x: 300, y: 70 },
    labels: [{
        markup: [
            {
                tagName: 'rect',
                selector: 'labelBody'
            }, {
                tagName: 'text',
                selector: 'labelText'
            }
        ],
        attrs: {
            labelText: {
                text: 'Label',
                pointerEvents: 'none',
            },
            labelBody: {
                ref: 'text',
                width: 'calc(w)',
                height: 'calc(h)',
                fill: '#ffffff',
                stroke: 'black',
                event: 'link:label:pointerdown',
            }
        },
    }]
});

were not triggered after the pointerdown event was executed.