Scannable cites (against 3.0)

This set of changes implements an alternative to the RTF Scan feature. It supercedes #172.

The changes add an option to the Export preferences that alters the behaviour of a cite drag-and-drop or Ctl-Alt-A capture to produce a scannable cite using the currently selected style. The cites contain a zotero:// protocol link, which ties the cite to the original Zotero entry, both for scanning and for clickability in external note-taking, outlining and word processing tools. Scannable cites can be converted to Zotero references in an ODF document using this Python script: https://github.com/fbennett/zodfscan

There are quite a few changes, so this isn't a serious proposal for 3.0, but the patch can be used to try out the approach for bugs and usability. There are a few blemishes:

* I have had a report that the self-destructing window fails sometimes on a Mac system;
* The option in Export preferences is maybe not the ideal means of controlling the behaviour; and
* The conversion tool assumes that a document with existing Zotero cites has been set to
  an in-text style before conversion, and an in-text style should be selected on the first document
  refresh.

Apart from that, it seems to work quite well.

(Sorry for the resubmit -- this patch was prepared against the 3.0 branch, but I mistakenly sent the original pull request to master.)