This adds support for snapping for line gesture tool. It is implemented in the modal keymap as Snap, which is a toggle (similar to how snapping in the transform operator works). Right now it snaps the angle of the line to 15 degree increments, which is defined in code. This should be easy to expose in the UI in the future if we need to. Reviewed By: Severin Differential Revision: https://developer.blender.org/D9115