This adds support for line gesture to SculptGestureContext and implements a Mask Line tool, which affects everything to the right of a plane defined by the straightline gesture. For this to work, a new WM_gesture_straightline_oneshot_modal is needed which only runs exec when the gesture is over. Added as experimental as it does not have icon. Reviewed By: Severin Differential Revision: https://developer.blender.org/D8722