Fix placeholder not carrying base item class

[pablosichert]

Hey there,

this PR fixes a bug when using the itemClass option and dragging an item. The placeholder also needs the default itemClass (to be consistent with the behavior of items placed on the grid).

[adumesny]

thanks for the fix. I supposed if you set opts.itemClass to something other than grid-stack-item we still need grid-stack-item in the DOM as the CSS is still dependent on that. I think addWidth() will also need to add this.ops.itemClass to this code

doc.body.innerHTML = '<div class="grid-stack-item"><div class="grid-stack-item-content">${content}</div></div>';

if you don't mind doing that and testing that out.

[pablosichert]

addWidget

calls

makeWidget

calls

_prepareElement

which sets this.opts.itemClass.

So I think it should be fine now without further modifications.