Add support for clipping edges at subgraph border

[hlindberg]

To draw an edge between two graphs an edge is required between nodes in the respective graphs with clipping set to the graph edge. In dot notation this is:

compound=true;
subgraph cluster_A {
    a;
}
subgraph cluster_B {
    b;
}
a -> b [lhead=cluster_B, ltail=cluster_A];

Support for this should be added by introducing the two styles "clipHead" and "clipTail" applicable to Edge. They require a Graph as value.

[hlindberg]

clipping is already part of dot, and is used for clipping head/tail to the perimeter of a node shape (which is the default, but it can be made to go to the center of a node shape instead).

Implemented as "toCluster" and "fromCluster", and when a style is applied there is a check that the given cluster is actually a parent of the applicable edge's node.