Merge branch 'master' of https://github.com/cbhaley/calibre

Bulk metadata dialog: Fix search and replace on datetime fields
failing with an error.
Update the metadata download and Get Books plugins for ozon.ru

Changes in OZON.RU corresponding files due to API- and HTMLlayout
Changes from ozon.ru Addition improved search hit relevance calculation,
introduced configuration parameter.
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Escape search string when showing message about searching.
Modify dirtied question if many dirtied editors present.

Also mark editors as clean when updating editors from container.
Make saving spinner not resize the status bar
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Remove references to Sigil from the manual and code
Implement saving of copies
Remove per editor saves as they cannot be implemented robustly

Also, show busy cursor when running any potentially long winded job.

Now that we have gotten rid of per editor saves, we track sync status of
editor with container, instead, and when performing any task that
operates on the container directly, sync open editors to the container.
This avoids the confusing warning about modified files.
Use the IDNA encoding for non-ascii hostnames
Keep list of recently opened books
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Right click to export file from book to disk
Nicer error message for DRMed books
Implement replacing of files
Hide the new Twwak Book behind a testing tweak
Allow customizing the text that appears below covers in the cover grid

You can now show any metadata field below the covers in the cover grid
view, not just title. To change the displayed field go to
Preferences->Look & Feel->Cover Grid.
Preload embedded fonts so that the preview panel works off the bat
Fix editing of .txt files broken