change icon for one instance of a actor/work object

[hofstef]

Is your feature request related to a problem? Please describe.
One of the ideas of domain storytelling's pictographic language is to use domain-specific icon. But so far, only a fixed icon set is available in the modeler. With this feature, it would be possible to add individual icons (or even more complex graphics).

Describe the solution you'd like
Change an icon by uploading a SVG from a local drive.
The custom SVG is exported in the .dst file to ensure compatibility.

Describe alternatives you've considered
Might not be that important anymore after implementing #32

Additional context

[hofstef]

We will provide a customizable icon set (#32) which seems to make this idea obsolete.