Allow long(er) labels

[FeldrinH]

Currently it seems that after 128 characters the label of a node/edge is truncated and the rest of the text replaced with .... This limit appears to be hard coded (the truncation is implemented here).

I currently need to have longer labels than this and have run into this limit. It would be nice if the limit was removed (I think a library user should have full control of their graph and can truncate labels themselves if needed). If removing it seems too dangerous then at least the limit could be increased to a much bigger number, for example 1024.