Add semilandmarks support to the mesh warping UI

[adamkewley]

Estimate: 10-40d (uncertainty is because we haven't researched into it much yet)

The thin-plate spline (TPS) algorithm work in #467 produced a UI that can help with landmarking + warping a single mesh. However, it does not support "semilandmarks", which is what the research generally refers to.

Semilandmarks are an extension of TPS in which the user defines semi-landmarks that are allowed to move along the landmark-to-landmark line. Their flexibility tends to lead to better warping results (citation needed).

There is an R codebase that features warping (need to find it), and I have a copy of PG's mathematica scripts, plus there's published work that is related to semilandmarks available online.

[adamkewley]

Better for 0.5.1: not MVP for frame definition