Sculpt Gesture Tools: add a modal keybind that toggles Front Faces Only

Right now the only modal operation you can make while drawing a shape with the Box/Lasso/Polyline gesture tools is to move them by holding space.

I think it would be useful to have a new modal keybinding for toggling Front Faces Only while we're in the middle of drawing the mask shape. The only way to toggle it now is going to the tool and clicking the checkbox, or making a separate keybinding with Front Faces checked

Example: the mask for Suzanne's nose doesn't need to shoot through the mesh, but the one for the ear does. With a modal toggle it would be really quick to draw both of these masks.

All of this would also apply to the Face Set gesture tools.