[plugin_installer] Add automatic updates
This adds automatic updates for plugins, fixes #325 (closed).
Automatic updates are disabled by default. When plugin updates are available, there will be a checkbox to update plugins automatically next time. Successful automatic updates will show a message, which can be suppressed if wanted.
This MR is technically !60 (merged), but adapted to upstream changes.
Automatic updates can easily be tested by lowering the plugin version of any other plugin in
manifest.ini. Restarting Gajim with auto update enabled will re-download the plugin.