Ensure we know which cell we clicked

Cells in the grid are addressed by their pos-x, pos-y attributes. Find
these attributes by climbing the parents of an SVG node.
Create a configurable context menu

This is a context menu for the program grid. Items are grouped by
related items and displayed as a set of arcs in a radial configuration.
Each option is a triplet of the style, icon and callback that sets the
requested state for the program.
Add context menu for changing the world..
Make context menu disappear after selection
Make mem an instance variable

mem is no longer private/shared, but this is better form and makes it
less risky to use ProgramEditor instances.
Fix an attribute error