This commit adds an "Act as Administrator" toggle action to the
View menu if kio-admin is installed. The action allows switching
between acting as an admin with root-access or not.

This was already possible in Dolphin when kio-admin is installed by
editing the location bar directly. However this is somewhat
unintuitive and there are no warnings at all about the dangers of
acting as an administrator.

This commit adds a warning dialog when triggering the action. It is
somewhat explicit about the risks because this is in fact very
dangerous.

Furthermore, while acting on a view with administrative privileges,
a bar above the view shows up that contains a warning. The bar can
be closed to stop acting with elevated privileges.

The warning dialog can be disabled and re-enabled from the Dolphin
settings but only if the action is even available.

There is a lot more to be done to further improve this feature both
security-wise as well as when it comes to usability. But
...