The whole "if korganizer or kontact is running, bring it to the front, otherwise
start korganizer" logic becomes a single line of code.
I'm not 100% sure about the Windows code using QProcess; clearly it
relies on DBus anyway so the new way should work, but it mentions
korganizer-mobile.sh (which could be called by the DBus service file then...)

Click "Check My Calendar" in an invitation email.
In KMail, it starts standalone korganizer.
In Kontact, it switches to the calendar plugin.