Icons of drop targets

[hski-github]

In the mod manifest you can define the icon of a drop target. This is very helpful for usability, because you guide the user with the icon, what to expect to happen when using. This is an important part of the ease of use of Spotfire.

When developing a mod, it would be good to have the icons of the drop targets being available somewhere as part of documentation, so that you can think about, what would be best fit visually for your axis.

More important, it would be very useful if a mod could define its own drop target icons as svg images and add them to the mod like the icon svg, because new and innovative mods are visualizing data in a different way then the classic visualizations and then the classic icons don't fit.