add interface to change penwidth
add support for edge coloring

leave text color of nodes always black if not heated
qt_dotgraph: refactor edge coloring

 * take color as parameter, not obscure "heat" param
 * split up coloring of nodes and text
 * leave text color of nodes black if not specified otherwise
 * cleanup
Merge pull request #40 from forouher/feature/indigo/edge_coloring

qt_dotgraph: Two enhancements for a rqt_graph improvement