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.

Test Plan:
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

Tags: Dolphin

Differential Revision: https://phabricator.kde.org/D12732