#3470973: Retain the placement of components within the preview when inserting a component

Improving the styling that occurs when components are drag and dropped by:

• turning the ghost element within the preview into a blue line that's absolutely positioned to the top of a slot or root layout;
• displaying the clone element at the original place of the component with lower opacity.

As a bonus, I also managed to improve the threshold we have on drop targets when placing components into new slots.