Connectivity widget: move name list creating function up

This position seams more logical. Additionally, the list is now added to
the DOM before the table. Up to now it was added to the beginning of the
table element.
Connectivity widget: make current neurons selectable

This commit adds checkboxes to the list of the neurons currently looked
at. If the connectivity widget is used as a skeleton source it will now
return only skeleton models for the current neurons that have their
corresponding checkboxes checked (like in the tables).

Additionally styles are added so that longer neuron names wrap properly.

This fixes issue #584.