refactor(dia.Paper)!: change the default connection point to 'boundary'

[kumilingus]

Description

Change the value of the defaultConnectionPoint option to boundary to make the diagrams more visually appealing by default.

Migration guide

To use the bbox connection point set the defaultConnectionPoint paper option as shown below:

new dia.Paper({
    /* ... */
    defaultConnectionPoint: { name: 'bbox' }
});