Connectivity widget: have separate up and down thresholds for all neurons

Having control over up and downstream thresholds of the individual neurons,
helps to reduce the current set of partners to those of most interest.

This relates to issue #611.
Connectivity widget: add sum thresholds

Before there were individual thresholds per neuron, the threshold targeted only
the summed synapse count. This can be handy and this commit adds a up and
downstream threshold. The controls are are added as a footer row of the neuron
list table.

This relates to issue #611.
Connectivity widget: redraw tables when thresholds change
Connectivity widget: move neuron list slightly to the right
Connectivity widget: add option to hide single node partners

Oftentimes, single node partners are placeholders for actual neurons
which haven't been traced, yet. This option allows a user to hide them
by checking a checkbox.
Connectivity widget: add index number to neuron list

When there is more than one neuron selected, the synapse count columns
are separated to show the counts of the individual neurons and the sum.
The sub-columns are numbered and refer to the neuron list. To make this
clearer, this commit adds this number to every entry in the neuron
list/table is numbered as well.
Connectivity widget: rename threshold columns and make them slimmer

Due to their title, the threshold selecting columns in the neuron list
were relatively wide. This commit makes the title a bit cleaner by
renaming them from "Threshold Up" and "Threshold Down" to "Upstream
Threshold" and "Downstream Threshold". At the same time, the width of
the column is restricted so that a line break is inserted.