Only a single `select` prop was exposed, and this was set to "bezt.f1"
(so it affected the left handle only).
Now we add properties for `left`, `right` and the `single` handle, the
selection prop for the control point is renamed to
`select_control_point` [which is now in line with the API for a bezier
curve].
NOTE: setting/getting a `single` handle uses the existing functions for
this -- handle is selected if any of the right/left handle is selected,
setting the handle results in both handles getting selected.
Pull Request: https://projects.blender.org/blender/blender/pulls/135114
462d887114