Allow holes in paths

[pasbi]

Draw a circular path and another circular path completely within the first one (both paths are not connected but belong to the same PathObject).

Commonly, the object will be rendered like a ring, i.e., a disk with a hole.
This is currently not possible, but it should be.

It was anticipated that #218 implements this feature.
However, the PR became too complex and now only prepares this feature.

Challenges:

• identify faces.
  There's currently no way to name faces or to identify them. Enumerating only the PathPoints around does not suffice. E.g., F1 and F2 are distinct but surrounded only by points A and B:

  ____
 / F1 \
A------B
 \ F2 /
  \__/

• design an appealing user interface.
  The number of faces can become quite large and identifying faces with text might be a problem.
  Presumably, an interactive selection in the viewport is required.