PP: Improve drag-and-drop UX

[dolsysmith]

The drag-and-drop feels a tag glitchy, and it's intuitive neither where to click to start dragging the element, nor where to place it on the drop target.

The Mozilla article on the Drag-and-Drop API has some suggestions under the "Drop Effects" section.

[dolsysmith]

Especially with longer blocks, the glitchiness is evident.

• When dragging some blocks, it looks as though the text itself is moving, not the DIV.
• When trying to move a block, sometimes it seems that multiple blocks are being picked up.
• This also causes the highlighting to "bleed" through ... the dragover class appears to get stuck.

[dolsysmith]

With Max's modifications, mostly working, but after a lot of drag/drop activity, it will start to act buggy. Do more testing: it may be that normal use will not trigger this behavior.

[dolsysmith]

Add <span> elements to enclose text.