Instantiate of abstract MVRScript's when using *.cslist's

[MacGruber]

In some cases it would make sense to have an abstract base class. For example an improved script base class that provides some utility methods, derived from MVRScript. It would need to be abstract, so it does not actually create a script instance. Some other class in the *.cslist would derive from it and provide the actual script instance.

Currently trying something like that causes an error message.

This should be a simple check for ScriptType.RawType.IsAbstract in MVRPluginManager.SyncPluginUrl before calling CreateScriptController (two locations). The warning "No MVRScript types found" would need to be triggered also when the array turns out to be only containing abstract classes.