startkde: start D-Bus session bus correctly

I had the wrong path to dbus-launch before; we were running KDE on the
system bus! Also, the session bus needs to be started before the splash
screen if the progress bar is going to work.
startkde: set XCURSOR_PATH properly

Cursor themes are now available in System Settings, and the theme is set
correctly for applications and the desktop. Plasmoids still have the Qt
fallback cursor theme, indicating that Plasmoids get started with the
wrong environment. That probably also explains why timezones aren't
detected correctly: TZDIR isn't set in the Plasmoids' environment!
startkde: set default cursor earlier