Improved display of plain text messages and text to HTML conversion (#1488937)

Now instead of <pre> we use <div class="pre"> styled with monospace
font. We replace whitespace characters with non-breaking spaces where
needed. I.e. plain text is always unwrappable, until it uses format=flowed,
in such a case only flowed paragraphs are wrappable.

Also conversion of text to HTML in compose editor was modified in the same way.
Rephrase editor warning to better match both ways text2html and html2text
Improve _convert_line() performance
Fix flowed lines recognition
Disable wrapping non-flowed lines on dash character
Fix invalid closing tag
Merge branch 'dev-text2html' of github.com:roundcube/roundcubemail into dev-text2html
Bring back the old behaviour where text messages without format=flowed are auto-wrapped.
Make it the default in text2html class.
Small improvements to make treelist work in search-mode
Improved handling of new-lines in quoted paragraphs on text2html conversion
Plugin API: added imap_search_before hook
CS fixes
Update changelog
Minor fixes for teelist searching
Merge branch 'master' of github.com:roundcube/roundcubemail
Small css fixes, optimized some images
Merge branch 'master' of github.com:roundcube/roundcubemail
Add timeout for password-driver vpopmaild
Merge pull request #190 from simonpl/master

Add timeout for password-driver vpopmaild
Fix handling unicode characters in links (#1489898)