Show a warning when running as the root user
Summary: Now that Dolphin can be run as the root user again, let's show a warning.
When run with the root user account:
Reviewers: Dolphin, markg, elvisangelaccio
Reviewed By: markg, elvisangelaccio
Subscribers: acooligan, anthonyfieroni, chinmoyr, kfm-devel, rikmills, emmanuelp, zzag, nicolasfella, elvisangelaccio, Fuchs, mmustac, markg
Differential Revision: https://phabricator.kde.org/D12732