The Plasma Applet Browser is now integrated into the libplasma and it will be available in the next Beta that is going to be published on Wednesday.
I’ve replaced the old drag-and-drop with a bit fancier stuff - now (as you can see in the screenshot) when you drag the plasmoids, you will be shown only the icons (possibly will be replaced by screenshots of plasmoids) not the whole “blue bars” you saw last time.
At the moment, you can add only one applet at a time by dragging, but that will be changed soon. If you use Oxygen style, you’ll encounter a nasty bug concerning the drag-and-drop-icons - there will be no icons, only nice shades of gray… but that’s not my fault.