Add virtual to a lot of the destructors. Note: perhaps it will have to be reverted for some files to keep them C-compatible.

Layout algorithm is now applied to each connected component in the selection separately. Previously, behaviour of layout on disconnected graphs was... undefined!

Previously graph layout was done using the Kamada-Kawai layout algorithm implemented in Boost. I am replacing this with a custom implementation of a constrained stress-majorization algorithm. The stress-majorization algorithm is more robust and has better convergence characteristics than Kamada-Kawai, and also simple constraints can be placed on node position (for example, to enforce downward-pointing edges, non-overlap constraints, or cluster constraints). Another big advantage is that we no longer need Boost. I've tested the basic functionality, but I have yet to properly handle disconnected graphs or to properly scale the resulting layout. This commit also includes significant refactoring... the quadratic program solver - libvpsc (Variable Placement with Separation Constraints) has been moved to src/libvpsc and the actual graph layout algorithm is in libcola.