Enable copy innerHTML by parsing outerHTML

The markup-view "Copy inner HTML" context menu doesn't work right now as the `innerHTML` walker method is marked as TODO.
The downstream protocol doesn't support `getInnerHTML` unfortunately, but I reckon we can use `getOuterHTML`, parse the result and get the inner HTML from that.
The attached code should work in most cases.
This is an unrelated change to this PR, just to clean up things a bit.