run plugin_downbloaded callback in the main thread instead of in Ftp Thread. Fixes freeze under windows when plugin is downloaded

improve loading of muc participand list speed by showing it only when we get our presence (this means we are connected). Fixes #6950

follow RFC-6120 section 6.4.4: send <abord> and a SASL authentication mechanism fails, wait for server <failure> reply before trying a new mechanism

merge XEP-0198 implementation from gajim-XEP-198 branch to trunk. Thanks Jefry for your work during GSOC

check DIGEST-MD% and SCRAM-SHA-1 latest answer from server before accepting authentication. Fixes #6940