Enable scripting the appearance of objects

It would be nice if the appearance of objects was not solely determined by a few hardcoded options, but could be replaced or added to by scripts.

A script could for example decide to render some primitive based on a custom property, like displaying a radius or a vector.

In some cases it would even just be useful if the script could determine the color of the object (see [this forum thread](https://discourse.mapeditor.org/t/is-it-passable-to-add-colour-to-enum/6854)).

Expansion of #949.

Start by reviewing the earlier work in issue #949 and the linked forum thread about coloring enum values. Then trace how Tiled currently chooses object appearance and where scripts can participate. Done should be defined as a documented scripting mechanism that supports custom appearance decisions, including the requested color use case.

Written by the indexing model from the issue text.