Hook access to geometry selection

[c42f]

When a vertex is clicked on, it would be useful to be able to get the selection as a hookable event, and to get the geometry label and vertex which was selected.

[c42f]

@Ross-Batten

[c42f]

Possible design:

$ displaz -hook select:vertex selected_geometry_index
select:vertex "label1" 0
select:vertex "label2" 1
select:vertex "label7" 100
...

That is, the payload selected_geometry_index returns the geometry label and the selected (0-based) index in the original dataset (ie, the index of the dataset as loaded, not as internally reordered by displaz). Using the label in this scheme is a little annoying in that it'll have to be quoted, or the spaces escaped.