[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.