Object Tree: "Cleanup fragments" popup menu entry
which prompts for a minimal size in skeleton treenodes
for skeletons to be sent to a neuron under the 'Fragments' folder.
The new neuron is named "Fragments" plus the date.
If the small skeleton exists in a neuron that only has one skeleton,
that whole neuron is sent 'Fragments' group.
graph2: fix error when splitting both by confidence and by synapse clustering
when there is a single neuron to expand by synapse clustering
and none other to split by confidence.
Graph widget: more robust way of detecting what kind of data the server sent.