feat(dia.Element): port position args now provided inside position property

[zbynekstara]

Description

Currently, port position arguments are read from top-level args port property, which is inconsistent with port groups and port labels (they both read those from position.args property).

This PR aligns the three - port's position.args are the source of truth for port position arguments. (To keep backwards compatibility, if there are no position.args, args are checked second.) For example:

g1.addPort({
    position: {
        args: {
            dx: 20
        }
    },
    attrs: {
        circle: {
            magnet: true,
            stroke: '#31d0c6',
            'stroke-width': 2,
            fill: '#ffffff'
        }
    }
});