Remove now unused ProbeInterface argument from createWidget().

With the UI running in a different process now, there's no access to the
probe anymore anyway.
Start to split the tool probe and UI interface.
Move the tool plugin model into the server.

Besides actually showing the plugins found on the target this is one step
towards isolating PluginManager inside the ToolModel, which will simplify
splitting it into a probe and UI part.
Also move the plugin error model to the server side.

This also removes the last reference to PluginManager outside of the tool
model itself.