Improve handling of multi-line node/edge labels.

Graphviz stores each line separately in u.txt.para, therefore RGraphviz only
returned the first line. When using the common workaround of \\\n in the
label, it did not get layouted properly and the codepath for labelsize > nodesize
kicked in, decreasing overall font size, which is very annoying when using
fixedsize=FALSE.
Re-layout the graph to the current plotting region when plotting.

This eliminates the problem with edges going inside circular nodes (which
was due to the fact that symbols always takes the x axis units for radii,
which problematic for non-square plotting regions) and makes working with
fontsizes easier.