qtui: Fix initialization order after switching to PMF connects

Seems like switching to PMF connections has changed the order signals
are processed in, resulting in the UI being initialized after the
session manager asks for saving the UI state (for whatever reason).

Make this less fragile (and avoid a crash when using KDE) by
connecting to the session manager only after initializing the UI, and
replace the initUi() slot by a lambda while we're at it.