Transform Modal Map: Toggle Plane

In other 3D tools I've used it is possible to change the transform tool's behaviour to use the plane normal to the held handle (usually by holding shift as a modifier).

This is especially useful for scaling both directions simultaneously in the plane of a selection.

Currently if I want to scale in the plane I have to switch to the scale tool and drag the appropriate plane handle. This functionality essentially permits this while already dragging a handle with the transform tool.

If dragging a resize handle + shift the scale should occur in the plane perpendicular to the handle. If dragging a translate handle then the translation should occur in the plane perpendicular to the handle (e.g. average normal vector of selection if orientation is normal).