Ported from kapp to KInstance+QApplication. We need still kdeui for kstartupinfo, but this way we avoid:
- the dbus registration
- a lot of other gui init done by kapp
- useless kapp command line options (kstart --icon foo ... won't do the expected)
- the startupid env reset by kapp, which we then need to set again before starting the app (cc to Lubos for post-commit validation of that change)

Also turned the KProcess into a pointer to avoid 'QSocketNotifier: Can only be used with threads started with QThread' warning.