Adding Bubok Spain store to the search plugins

Bubok Spain is a publisher, store and library with a big book data base
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Get Books: Add a new store, Bubok (bubok.es)

Merge branch 'master' of https://github.com/bubok/calibre
Merge branch 'master' of https://github.com/j-howell/calibre
Fix #1386513 [Little typo? (semantics)](https://bugs.launchpad.net/calibre/+bug/1386513)
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Implement API to get menu layout and properties
Tweak to allow minimum size for proceed popup