When zoomed out, cannot drag an element beyond canvas extents

When zoomed out, the user can see past the end of the canvas without any visual indication. Trying to drag an element onto seemingly-legal white space is disallowed by the editor since it's past the edge of the canvas.

suggested fix: when zoomed out, allow dragging past the edges of the canvas, and when the element is dropped, automatically expand the canvas to include the dropped element. (c.f. OmniGraffle's behavior when the "auto-expand canvas" box is checked)