ENYO-2151: Bad cursor style applying fixed as the mouse leaves a node by...

... the right and goes to another one

In this case after the drop, DnD applied style is not removed to the leaved node and as the mouse cursor go freely on it, the DnD style is still shown
Addition of a background decoration to indicate that the folder parent node above can receive the drop as the cursor is dragged to the left of a file/folder node
Code cleaning in order to better integrate future file manager improvements