Localization: property files for strings generated from scripts

Almost all the messages are now localized (and translated into French).
I've tested the main strings (but some are really difficult/impossible to get for me) but not extensively. You'll see that I wasn't very sure of when I should use <code>this.ZFgetString('xxx')</code> instead of <code>Zotero.ZotFile.ZFgetString('xxx')</code>...