Improve sudo/kdesu error message
Now that you can get elevated privileges while using the app, we can
display a more useful and descriptive error message when people try to
run Dolphin using sudo
or kdesu
.
Also bump the frameworks dependency to 5.90 since that's the KIO version that includes this change, so we can be sure that the message is always accurate.
Not to be merged before Frameworks 5.90 is tagged to avoid issues with the CI.
cc @teams/vdg