Use the webservice to download data asynchronously

The plugins list and all the plugins are downloaded via this method.

A msgbox is showed once a plugin is successfully installed, and is
removed from the available list.
Added Update plugin capability

Before updating, the user is warned if the local plugin files have been
edited.

The only thing that's left now is to move the common methods b/w
plugins.py and plugins_download.py to a single file.