Don't output unnecessary Group Blocks in list items when copy pasting HTML content in WYSIWYG

If the HTML copied has list items that contain single blocks (e.g. images) wrapped inside a DIV, remove the unnecessary DIVs by using a filter.

Example HTML:

<ul>
<li><div><img src="imagesrc"/></li>
</ul>

Example resulting xwiki syntax:

* (((
localimagesrc
)))

Expected:

* localimagesrc