Plugin auto-load option: "no entry in PV plugin manager"

Own plugins can be written and directly integrated into a custom application. Loading them can be automatic with the REQUIRED_PLUGINS list of the paraview_client_add cmake function or inside the program using the vtkPVPluginTracker class.

In all these cases, the plugins will appear in the list of the plugin manager - as "loaded".

But this does not make too much sense because loading them is not any more required, and unloading makes not much sense (and I would even doubt whether it can be reliable to "unload" a shared library from a running program!?)

Having an an option during plugin auto-load to not show a plugin in the PV plugin manager makes sense thus and avoids filling the list with useless entries.