diagram-svg as a simplified / more robust version of the gongsvg component

[thomaspeugeot]

Steps:

gongsvg is:

• more complex that the preceding version because there is only one template.
• therefore more difficult to debug, espcialy since it is always redrawing

Steps:

• #115
• #117
• create a state variable for the gongsvg ("idle", "drawing a line", "dragging a rect", "dragging a link", "dragging an anchor", "drawing a link between rects", "selecting multi rects" that is explicit
• #119
• #121
• #123
• #125
• #127
• #129
• #131
• #133
• Rename as gongsvg-diagramming
• cleanup