[Retrieve Metadata] Recognize HTTP 503 code as Google Scholar CAPTCHA + other tweaks.
* Stop metadata retrieval when cancelled
* Display CAPTCHA dialog
* Don't close window on blur
* Use Zotero.Utilities.cleanISBN to validate ISBNs
[Retrieve Metadata] Look for JSTOR stable URLs and convert to DOIs
Thanks to myqlarson for the suggestion. See https://forums.zotero.org/discussion/29354/1/query-limit-reached/#Item_42
[Retrieve Metadata] Use a single queue to query Google Scholar. Window closing tweaks.
* Close window on blur after completion on Mac (revert previous change)
* Don't close window when canceling
* Add Esc handler to cancel/close window
* Allow columns to be resized
* Fixes #445
* Fixes #444
[Retrieve Metadata] Focus item in Zotero pane when double-clicking
* Fixes #446
[Retrieve Metadata] Add/fix comments
Merge pull request #433 from aurimasv/retrieve-meta

Retrieve Metadata query limit fixes