option to use Zotero rename rule for naming files

[keighrim]

This PR replaces citeKey pref with files.basename pref, and makes it a ternary option by adding "use zotero rename rule" value. This can be further expanded to include more options like re-use the zotfile rule or custom format (in a "printf" style), but this works well enough for me for now.

Also, the base name formatting is moved to the Files names tab in the preferences window, putting it among other naming options.

Few things that are still missing;

1. localization other than en-US
2. migration from exiting a citeKey value (bool) to new files.basename value (string).

Let me know what you think.