Connect postsynaptic treenodes with connectors

When a treenode is activated, alt-shift clicking on an existing
connector creates a link with the node postsynaptic to the connector.
This allows interface parity with creating a new connector to which
the activated node is postsynaptic by alt-shift clicking in empty
areas. Closes acardona/CATMAID#712.
Only load tiles within slice boundaries

When artificially zoomed, clamp tile loading within slice boundaries
to prevent nonexistent tiles from being requested from the datastore.
Closes acardona/CATMAID#719.
Merge pull request #716 from aschampion/issues/712-presynaptic-connector-assignment

Connect postsynaptic treenodes with existing connectors via alt-shift click
Merge branch 'flow-centrality'
Merge pull request #720 from aschampion/issues/719-zoom-tile-boundaries

Only load tiles within slice boundaries
Connectivity widget: add missing parenthesis and quote to tooltip

The tooltip is shown if one hovers with the mouse cursor over the
synapse count table cell for one of multiple partner neurons. The change
relates to issue #721.
Connectivity widget: show neuron name service name in tooltip

The tooltip is shown if one hovers with the mouse cursor over the
synapse count table cell for one of multiple partner neurons. The change
closes #721.
Neuron name service: add refresh method

This method is a convenience method to update all name representations
and notify all registered widgets in one go.
Neuron annotations: refresh name service when annotation is added or removed

Now the neueon name service is refreshed (and all clients are notified)
as soon as an annotation is added or removed. This fixes #718 and
relates to #717.
WebGLApplication: new function Skeleton.splitByFlowCentrality
returning a map of node ID vs true for nodes that belong to the axon.
WebGLApplication: re-implement color connectors by axon and dendrite using Skeleton.splitByFlowCentrality.
WebGLApplication: new skeleton coloring scheme "Axon and dendrite"
using Skeleton.splitByFlowCentrality, with:

axon: green
dendrite: blue

... the same colors that coloring connectors by axon and dendrite uses.

This coloring scheme provides a visualization of the axon and dendrite domains
while retaining the pre/post identity of the synapses when colored normally
with red and cyan.