fix isOver on nested drop targets (fix #13)

[qtsd]

Current behavior :
- find all the drop targets that are under the mouse

Fixed behavior :
- take the first element under the mouse
- find any drop targets that are either this element or one of its parents
- sort them by their position in the DOM (from deep to shallow)

Here is a codesandbox to see the different behavior between html5 and mouse backends (to change the backend, see index.js).

[qtsd]

I realize this technique can have a flaw.
If you use a drag layer, it must have pointer-events: none, so the drop targets behind can be triggered normally.
Therefore it can be tricky to change the mouse cursor since with pointer-events: none, the cursor css rule is ignored.