Even though we passed the `class` attribute, it did not take it into
account. For example try to render this:
<$droppable class="custom-css-class">
... content ...
</$droppable>
Before applying this patch:
<span class=" tc-droppable">
... content ...
</span>
After applying:
<span class="custom-css-class tc-droppable">
... content ...
</span>
These changes allow drag and drop to work with one issue: <a> links are
not draggable; draggable divs, spans, buttons etc. seem to work fine.
There’s some issue with IE11 that I don’t understand.
For testing, you can force links to become spans by changing line 64 of
$:/core/modules/widgets/link.js to:
var domNode = this.document.createElement("span");
