Style change: unify indentation in connectivity widget
Connectivity widget: re-order UI creation method

This, along with adding more comments, is done to make the process of
creating the connectivity widget's user interface more readable.
Connectivity widget: add row to syn count column header

This new row currently only includes the cell 'Sum' to make clear that
all numbers in this column are aggregated values over all selected
neurons. Soon, more cells will be added to have synapse counts for every
selected neuron.

This relates to issue #601.