cppcheck: performance + variable type

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.