This implements transform modes for the transform tool and Elastic
Transform. This mode uses the Kelvinlets from elastic deform to apply
the transformation to the mesh, using the cursor radius to control the
elasticity falloff.
{F9269771}
In order for this to work, the transform tool now always works based on
delta from the previous position instead of delta from the initial
position. This allows to integrate the displacement of the Kelvinet in
multiple steps. This change will also be needed for future features like
plasticity collisions.
The UI is not final in order to not create conflicts with D9540. The
transform_mode property will also be in the topbar, as in D9540.