Improve app usability and behaviour

[RiboRings]

A few elements of the app could be improved:

1. Allow multiple node selection via single click instead of long press click (check multiselect arg)
2. Turn off edge selection by clicking (users should be able to select only nodes) (maybe by turning off selectEdge visEvents)
3. Centre network (currently positioned to the right side of the screen)
4. Prevent network from being cropped in the top and left borders (bottom and right corners behave ok)