ApplicationLauncherJob: show the Open With dialog if no service was passed

This replaces KRun::displayOpenWithDialog, which this commit deprecates.
It also ports KFileItemActions to ApplicationLauncherJob for the
open-with dialog cases. Notice the nice code path unification where
the service might or might not be null.

The code for the Windows-specific native open-with dialog is now shared
between krun.cpp and widgetsopenwithhandler.cpp, after moving to
widgetsopenwithhandler_win.cpp. Untested, obviously.