When a workplane is added to a face, let CAD Sketcher extract vertices, edges and even the origin from that face, to be used in the new sketch

[raindropsfromsky]

Motivation / Existing Problem
When we add a workplane to any face, its only virtue is that it is coplanar with the underlying face. But for all practical purposes, the workplane is unrelated to the underlying face (let's call it the "Base Face", or "BF" in short)

But there is a major shortcoming: We cannot make use of the vertices, edges and origin of the underlying face to construct a new shape. As a result, the new shape cannot be locked to the existing face. Thus, we lose a chance to draw new features with precision on the existing face!

Description
When a workplane is added to a face, all the vertices, edges and origin of the underlying face should be accessible to the user, so that he can use them to construct new geometry.

Just like how the knife tool works in Blender.

For example, we should be able to do the following:

1. Begin (or end) a line using an existing vertex of the BF
2. Snap a line to any point on an edge of the BF
3. Snap to the midpoint of an edge of the BF
4. Snap en edge to the origin of the BF
5. Draw a circle by using the origin of the BF as its center point

This feature is provided in FreeCAD, which is known as "extracting geometry".