editor.insertHtml pollutes editable with empty spans

[engineering-this]

Type of report

Bug

Provide detailed reproduction steps (if any)

1. Open https://codepen.io/anon/pen/QYZZOG
2. Create selection in editor as following: Foo {Bar}.
3. Press button insert below editor.

Expected result

Selected part is removed. And there is no empty element left.

Actual result

Empty span is left behind editor which is visible by red border.

Other details

These empty elements can't be focused (checked Chrome, Safari, Firefox), and they are removed on getData. They should be removed right away on insertHtml, because when styled they might be visible in editor.

https://github.com/ckeditor/ckeditor-dev/blob/002ec74de01c50d48e1ba2101785126a56407eeb/core/editable.js#L1719-L1723
☝️ This is where they ale splitted upon insertions, and when splitting leaves empty elements it's best place to clean them up.

[jswiderski]

This issue was originally reported by Oracle and can be observed when pseudo elements are used: https://codepen.io/j_swiderski/pen/mvzvMZ.