Generic snap gizmo to be used for different tools.
The Gizmo can be configured initially by the following properties:
- `"snap_elements_force"`, `"prev_point"`
The following properties can be read as return:
- `"location"`, `"normal"`, `"snap_elem_index"`
This property can be linked to another (tool_setting.snap_elements):
- `"snap_elements"`
And this 3 extra utilities have been added:
- `ED_gizmotypes_snap_3d_draw_util`,
- `ED_gizmotypes_snap_3d_context_get`,
- `ED_gizmotypes_snap_3d_update`.
Differential Revision: https://developer.blender.org/D7071