Spine Snapshot Additional subtext support

[Hendrik-code]

Add in Snapshot_Frame a dictionary mapping centroid label to additional caption text that is placed below the centroid label

So it says something like
C2
Fractured: yes
...

Probably make the subtext smaller but keep the color of the label

[robert-graf]

This is already a hidden feature.

Add "text_sag" to the Poi info dict

poi.info["text_sag"] = {POI-ID: x}

x is either
"text"
("text", x-cord)
("text", x-cord, y-cord)

POI-ID
is a tuple of int with the relevant ID.

TODO: Make a sensible way to define this and add proper documentation.