Updated Spanish translation
EWebView: Limit zoom-in/zoom-out to 0.1 .. 5.0 values

It doesn't make much sense to zoom below 0.1, same as zoom 5.0 is
quite large, thus no need to zoom indefinitely in both directions.
Updated Spanish translation
EMailFormatter: Use GOutputStream instead of CamelStream.
Remove obsolete white-papers.
Miscellaneous cleanups.
Update API documentation.
NEWS update for 3.11.91 release.
Post-release version bump.
Updated Galician translations
Updated Traditional Chinese translation(Hong Kong and Taiwan)
Add a comment for translators above "_Send Account Override"
Bug #725402 - Checking sender for multiple images causes hang

This was introduced around commit ef355cd5, when the caching of
search results were dropped. The new function was also buggy, thus
this commit fixes even that.

Namely done changes:
- cache email address book search results for 5 minutes
- do not stop searching when any book fails to open (which can be
  due to temporary unreachable server, for example)
- search in local books first
Updated Norwegian bokmål translation