Make styling non-global

[Quincunx271]

Right now, the different styles (NodeStyle, FlowStyle, ConnectionStyle) are set globally. It's conceivable that one might want to have multiple different graphs in a single application. Right now, they can't be styled differently.

I think we should make it possible to set the styles on just a single graph. We could considering allowing every node and connection to have their own styles, but that might be overkill.

[Quincunx271]

Note that individual nodes can already be styled via NodeDataModel::setNodeStyle(), but it otherwise defaults to the singleton style