Add vertex style

[Yvee1]

I am interested in modifying the style of vertices drawn in the vertices mode. What about adding the following methods to geometry_renderer.h? The setVertexShape would take a path (see #31), working similarly to ipe markers, though the stroke width is not "hardcoded" but instead can be controlled via setVertexStroke.
We could add predefined vertex shapes like the ones available in ipe (disks, squares, triangles...).

/// Sets the vertex stroke style.
virtual void setVertexStroke(Color color, double width) = 0;
/// Sets the vertex fill color.
virtual void setVertexFill(Color color) = 0;
/// Sets the vertex size.
virtual void setVertexSize(double size) = 0;
/// Sets the vertex shape.
virtual void setVertexShape(Path path) = 0;

The Qt renderer could simply draw the path with appropriate styling and size, and for ipe we could export a symbol matching the set style. Thoughts?