Run simprocs only on the selected subgraph

[rossduncan]

If a subgraph is highlighted in the left pane of the derivation view, then if a simproc is run it should act only on the selected subgraph and leave the rest alone.

[Hector Miller-Bakewell]

In the GUI: There is now the ability to extract the selected vertices from a graph (complete with trailing edges); from there one could apply the simproc to make a new theorem, then apply that to the original graph? It's a little long-winded, but only just, and makes things a lot clearer in review.

I'll have a look at putting the functionality into the Simproc API.