Fix session management for KApplication based applications.

- Call QGuiApplication::setFallbackSessionManagementEnabled(false)
  to prevent premature application exit
- Wire up the saveStateRequest() and  commitDataRequest() signals
  to the appropriate methods that had to be turned into slots first.
  Those methods were never even called, they were not ported properly.
- Cancel logout when the user decides to do that. A comment in the
  code was not sufficient to do that. (?!?!)

CCBUG: 354724