bumped PP core model to v1.0.0.12
bumped pp.project models to v1.0.0.12 (because core PP model changed)
bumped pp.isabelle model to v1.0.0.12 (because core PP model changed)
bumped pp.zeves model to v1.0.0.12 (because core PP model changed)
removed ProofDecor element

ProofDecor is a special case of ProofSeq (with 1 element). Because of that, it just complicates the code using Seq/Decor interchangeably..
removed ProofDecor usage
special icon for single-element ProofSeq (becomes ProofDecor)
Merge branch 'decor-remove'