remove unused config option `display_content_in_threadline`

this has been replaced long ago by setting `search.threadline.parts`
in theme-files.

cf issue #631
fix command line escaping for arguments containing spaces
Merge pull request #632 from yannrouillard/fix_args_with_space_handling

fix command line escaping for arguments containing spaces
add helper to check if path is below another

this is used in the Database manager
raise exception when adding msg no non-indexable path

i.e., one not below that of notmuch's root path
fix: make sure DBManager is initialized correctly...

and knows the path to notmuchs root dir
move check to add_message directly
adjust error message
handle errors caused by trying to index mails...

... not below notmuchs root path

issue #611
Merge branch '0.3.4-fix-611'
properly read stderr in call_command helper
handle errors of external cmds in MatchSdtoutAddressbook

issue #624
fix: call_cmd properly read return value from...

subprocess.CalledProcessError
introduce CompletionErrors

and use them to handle AddressbookError exceptions
raised by addressbook.lookup
handle CompletionErrors

this makes the CompleteEdit widget handle errors
raised during completions by passing them on to
a 'on_error' callback given to its constructor.

This is then used in UI.prompt.
tune CompleteEdit:

don't insert spaces in case the completions set is unary
(no results found)
docs for CompleteEdit
doc: CompletionErrors
Merge branch '0.3.4-fix-624'
pep8&pyflakes fixes

mostly automatically fixed