Refactor the property editor to use display strings from the model.

This removes the dependency on the variant string conversion code, which
we now only need probe-side. This allows us to split that up based on where
the types are coming from and push it into plugins, removing gui/widget
dependencies from the core probe.
Split out Util::SetTempValue, that's also needed client-side.

This should enable moving the rest of util.h back into the probe.