Add element button in editor should stay fixed on selected element

[grester]

Right now whenever you hover an element, the plus button to add an element moves to the hovered element.
This is a problem because when you move the cursor to click it, 99% of the time you hover another element and the button is gone.
The button instead should stay fixed with the element that is selected with the blue outline, so that you can actually click it consistently and predictably.

[grester]

Moved to issue #390