Add zero-weighted edge support (?) and zero-weighted edge color selection functionality in Settings

[oxy86]

SocNetV omits zero-valued edges during network analysis computations. Nevertheless, when opening weighted edge lists, SocNetV accepts edges with zero values/weights and draws them (although completely by mistake).

A SocNetV user informed me that they need this functionality (zero valued edges to be drawn) and on top of that to be able to select some color for this kind of edges.

We will implement this functionality in Settings but it will be explicitly declared that the functionality serves only for those that have data (i.e. weighted edge lists) with zero weights on some edges and they still need to draw those edges, despite the fact that they will not be considered in any SNA computation by SocNetV.

Note: This functionality needs investigation as to what formats (except edge lists) support zero-valued edges.

[oxy86]

Last commit adds functionality in Settings for zero-weighted edge color selection.