This adds two new python properties to `GreasePencilStrokePoint`
* `select_handle_left`: The selection of the left handle of this point
* `select_handle_right`: The selection of the right handle of this point
These could already be written to using `drawing.attributes[".selection_handle_left"]`
and `drawing.attributes[".selection_handle_right"]`, but this makes it
easier to work on individual points from the higher-level
python API.
Resolves#137639.
Pull Request: https://projects.blender.org/blender/blender/pulls/137730
ee6d683809