For selected elements with path data, merges the paths by uniting, producing a path that fills the area that both the paths covered. Each merge will be applied to the selections sequentially. Merges will be effective on the front-most element.
A selected element will be filtered if the element is not a `path` element, has no `d` attribute, or has children. The merge will keep only the front-most element's attributes, omitting any attributes applied to other elements.
```sh
# Effects: History, Document, Selection
-path-unite
```