Neuron annotations: add dedicated method for annotation query creation
Neuron annotations: add query for annotations based on skeleton ID
Factor out skeleton model creation in own function
Skeleton: factor out arbor creation into dedicated method
Skeleton splitting: add basic dialog to front-end

It shows the split in a 3D viewer and gives the user the option to distribute
the existing annotations on both new sub-skeletons.
Neuron annotations: factor out annotation creation

The actual annotation creation now has its own method. This simplifies re-use of
it.
Skeleton splitting: add annotation based splitting

When splitting the front-end and the back-end now require that one part
maintains all previous annotations. The first part is referred to as 'over' and
the second one is called 'under'. The under skeleton will now be a model of a
newly created skeleton which also gets all the under annotations.