New constraint solver and softbody dynamics clean up

+ New constraint solver
    + Improve method to construct LCP problem
        + It does not require explicit skeleton's mass matrix and inverse mass matrix
    + Add constrained group concept (similar to ODE's Island concept)
+ Softbody dynamics clean up
    + Merge `SoftWorld` into `World`
    + Merge `SoftSkeleton` into `Skeleton`
    + Merge `SoftFCLMeshCollisionDetector` into `FCLMeshCollisionDetector`
    + Merge `SoftFileParser` into `SkelFileParser`

Note: There is more work to be done but I'm splitting it here. The API is not stable yet.