Circuit editor Z-index fixes

[stewdio]

Currently the circuit editor interface uses fancy z-index / ::before / ::after CSS maneuvers to create coherent outlines and shadows for complex selection shapes. But some inconsistencies remain. Example: Operations along the top register or the first moment when selected lose their upper or left edge outlines because the header cell borders take precedence.

In general the overall strategy of z-index use needs to be given a closer look. Obviously the user-dragging clipboard needs to be the highest z-index—even above the drop-down menu as it is rendered in mobile mode. But situations like rendering the circuit editor and Bloch Sphere visualizer on the same page offer more complex sorting situations.

Investigate the current uses of z-index. Come up with a strategy for organizing z-index “zones” for various uses. Implement.