Fix footnote font irregularities

Scrivener users [have been reporting](https://forums.zotero.org/discussion/29308/5/rtfodf-scan-for-zotero/#Comment_181805) that the font families and font sizes in footnotes can go askew after conversion. The problem seems to be  that:
* Scrivener explicitly sets one of the document's internal styles as text:span styling in footnotes;
* The RTF/ODF Scan plugin removes these styling spans in footnotes that contain only a reference;
* LibreOffice then stumbles on these notes, setting them to the global (not footnote-specific) document defaults.
* This misinterpretation of default settings then cascades through subsequent notes, affecting some (or all?) notes that previously had correctly assigned internal styling spans.
* The mixture of formatting parameters breaks styling control, so the mess needs to be cleaned up manually for each cite, after each regeneration of the ODF document.

This changeset creates a DOM from the document XML after performing the regexp changes, examines the text of footnote nodes for style declarations, and removes font family and font size attributes from these styles (only) in the document header, serializing the result as the final `content.xml`. This removes the Scrivener "hints" for these styles, with the result that LibreOffice imposes its own (consistent) defaults for font family and font size in the footnotes.

The stripping is limited to font family and font size, so it does not affect other parameters (such as italics), and font-family and font-size parameters can be controlled globally through the styles editor.

The modified plugin has been tested by two users (one [via the Zotero forums](https://forums.zotero.org/discussion/29308/5/rtfodf-scan-for-zotero/#Comment_201951)), who report success.