Display dialog to locate missing attachment files

[dstillman]

If a file can't be found, display a file dialog to let the user locate the missing file

Also worth investigating filesystem links, but I don't think we have too many options there (cross-platform or otherwise).

[dstillman]

(In [812]) Addresses #367, Handle missing attachment files

• Catch errors trying to display missing files and display message to user

• Switch to persistent descriptors rather than relative paths for attachment paths -- this will fix attachments on networked drives (which, at least on Windows, were not working and apparently in some cases breaking entire Zotero installs), but since WebBrowserPersist.saveDocument() is asynchronous and file.persistentDescriptor can't be set on Macs before the file exists, Attachments.importFromDocument() no longer returns the id of the new attachment, so translate.js had to be changed accordingly

• Try to convert relative descriptors to persistent ones with migration step (and later on access, if persistent fails)

• Added Zotero.WebProgressFinishListener(onDone)

Next step would be to throw up a file dialog to let the user locate the missing file

[dstillman]

(In [1189]) Closes #367, Display dialog to locate missing attachment files

New "Locate..." button in file not found dialog.

Split file not found message into two parts and updated localizations accordingly