Connectivity widget: show hidden partner stats in table footer

The connectivity allows to add various filters. One can, for instance,
add thresholds or one can hide partners with only one node. To make the
display consistent, this commit introduces a footer row to the
connectivity tables.  This footer displays the aggregated synapse
count(s), review percentage and node count of all hidden nodes. It's
naming and coloring makes it clear that it isn't referring to an actual
neuron.
Connectivity widget: center synapse count numbers

The synapse counts in the up and downstream tables are much more
readable if they are centered (or at least not left aligned).
Connectivity widget: center select checkboxes in tables cell
Connectivity widget: fix small table alignment bug

If both tables are being displayed next to each other, one has to make
sure they are vertically aligned to the top. It can otherwise happen,
that one table is displayed with a small gap on top, relative to the
other one.
Connectivity widget: make collapsing of tables faster

Instead of hiding/showing each table row individually, the complete
table body is not toggled. This improves performance considerably and
now very large tables can be collapsed in very short time as well.

This fixes #600.