Add session manager connection for KDE

[bluss]

This message shows up for anywhere we don't support the session manager.

[kupfer.ui.session] SessionClient: Warning: Not able to connect to current desktop session, please Quit before logout to save kupfer's data.

It may be needed to exit cleanly on session exit (logout, reboot). Kupfer saves data on SIGTERM as well, so if it gets that signal and some time, that's fine too.

[SanskritFritz]

Kupfer does get the sigterm signal, so it works, I can confirm this.
UPDATE: I mean in KDE for that matter.