Make the snap system consistent with the placement tool and leak-safe. **Changes:** - Store `SnapCursorDataIntern` in a `static` variable; - Initialize (lazily) `SnapCursorDataIntern` only once (for the keymap). - Move setup members of `V3DSnapCursorData` to a new struct `V3DSnapCursorState` - Merge `ED_view3d_cursor_snap_activate_point` and `ED_view3d_cursor_snap_activate_plane` into `state = ED_view3d_cursor_snap_active()` - Merge `ED_view3d_cursor_snap_deactivate_point` and `ED_view3d_cursor_snap_deactivate_plane` into `ED_view3d_cursor_snap_deactive(state)` - Be sure to free the snap context when closing via `ED_view3d_cursor_snap_exit` - Use RNA properties callbacks to update the properties of the `"Add Primitive Object"` operator