do not ask for confirmation to close PlayOnLinux if there's no program started

That may require further cleanups, as it revealed that mainwindow's
registeredPid list contains lots of obsolete PIDs.
Merge branch 'master' of github.com:PlayOnLinux/POL-POM-4
do not suggest bug reporting if a script has been canceled because a prefix already existed

Could be worth disabling also in case of prefix overwrite, as the state
of the installation is not reproducible.