[plugin_installer] Add plugin auto update

This adds an option to automatically update plugins (default). If there are updates, the affected plugins get downloaded and reactivated.

Auto-update option is only available if "Check for updates on start" is also activated.

Fixes #134 (closed)

Additionally, this sets the Install/Upgrade button's sensitivity to false. This prevents the button from being activated in a case where no plugin is selected.

Testing: Auto update can easily be tested by lowering the version number of one (any) installed plugin in the corresponding manifest.ini. Restarting Gajim with auto update enabled will re-download the plugin.