[PFW] Convert span background to background-color if possible

[Marek Lewandowski]

New PFW implementation puts font background as span with background css property. It's not handled by any of our features (colorbutton in particular), and therefore ACF strips it in the process.

If we'd set it as span background-color everything would work OK.

Actually it would be nice to reuse ​`config.colorButton_backStyle` (if possible), that way it will use a proper style, even if it was remapped to different style.

1. Open ​PFW full manual test case.
2. Open any document with font background in Word (e.g. last two lines in ckeditor-dev/tests/plugins/pastefromword/generated/_fixtures/Fonts/Fonts.docx).
3. Select some text with background in Word.
4. Copy it.
5. Paste into the manual test editor.

Expected: Pasted rich text preserves text background.

Expected: Background gets removed.

Reproduced on Chrome @ Win10.