Add graceful quit without closing stream for fast consecutive start
I suppose Gajim is being closed for (at least) following reasons:
- PC power off
- PC restart
- Graphical session restart
- Gajim update
- Gajim crash
All cases except the first one, last only for some seconds or minutes. This means there is actually no need to close the stream, so that Gajim can start much faster thanks to the stream resumption.
So once the stream has been established, the corresponding ID could be saved for the particular account. If someone choose to normally quit Gajim (e.g. for power off), the ID will be removed, otherwise ID will be left. So even if Gajim crashes the ID survives. When Gajim starts, it checks, whether stream ID exists, and if yes, tries to perform stream resumption.