Prefer GammaRay::UserRole over Qt::UserRole.

This fixes bugs when one connects a Qt4 client to a Qt5 server and
vice versa, as this enum's value changed. So now always use 256
or GammaRay::UserRole where applicable to prevent any future issues.

Also increase the protocol version to catch issues due to that now.
Make it possible to use ObjectVisualizer on servers without VTK.

Only the client/GUI needs VTK, the server-side model works fine
without it.

This is a bit hacky as it will now always show the Object Visualizer
tool and you can select it in clients, which where built without VTK.
Then an error message is shown. The proper way, of having the tool
disabled, won't work until we properly split client/server factories.

This also makes it possible to visualize objects in a Qt5 app, as
long as you use a Qt4 GammaRay client.