GNOME Shell integration for Chrome version 8.1

Changes

Added check to make sure that "enabled-extensions" GNOME Shell setting does not contains duplicated values (GNOME Shell bug 777650).
Fixed error that breaks update check if there was extension installed without "version" key in manifest.
Fixed error that prevents error message to be shown when update check is failed.
Fixed connector segfault on browser close.
Fixed possible error in synchronization process that may delete some extensions.
Switched Ubuntu PPA packages to use Python 3.

Native host connector is not working in Google Chrome and Opera when installed via Debian Stretch or Sid repositories. To know "why?" look here, here and here. For a workaround look here.
Web extension is broken on FreeBSD because of bug 212925.