Requires Qt 5.6 branch not more than a few days old, or >= 5.6.0
when it is released.
Parts of the fix are:
- Call QGuiApplication::setFallbackSessionManagementEnabled(false)
  to prevent application suicide through a mechanism that tries to
  help applications without any proper session management support,
  but badly interferes with applications that do implement proper
  session management, such as KDE applications.
- Add back commitData[Request] handling. For some reason it was
  removed during porting.
- Change the returned types of saveState() and commitData() to void.
  The return values were unused.

BUG: 354724