Fixed selection of connection

The connection 0 from source was always incorrectly selected. This no
longer happens. The connection 0 from source is only being highlighted
if the connection is selected.
Implemented bezier curves for synapses

Bezier curves implemented for sysnapses.
The control points are based on source and destination coordenates
Display list

Implemented display list for populations. Each population stores its own
dlIndex. Known bug -> when changing population display style, dl is not
recreated.