Constrain Assistant Editors to Viewport Bounds.

What: This commit constrains the position of each assistant editor widget within the bounds of the viewport, allowing the user to access and manipulate (move, hide, edit or delete) assistants that would typically be off-screen.

Why: This leads to a noticeable workflow improvement for precision drawing in at least two common scenarios; when zoomed in to do fine details in perspective, and more importantly, when dealing with distant vanishing points in subtle 2-point and 3-point perspective drawing.