Connectivity widget: add individual synapse count for selected neurons

The synapse count column is currently an aggregation over all synapses
of all selected neurons. This commit changes this by splitting this
column for individual neurons, if there is more than one selected. In
this case, the 'syn count' column will have sub columns for the sum and
the respective synapse count for the individual neurons.

This relates to issue #601.
Connectivity widget: add tool-tip for hovering syn count column

Hovering with the mouse over a synapse count cell will now produce a
tool-tip. It includes how many synapses there are for which selected
neuron. This tool-tip basically relates the number within the cell to a
neuron name list entry.

This relates to issue #601.