BALLView should warn on start when no renderer plugins are found

[anhi]

Reported by odin on 10 Aug 43379833 18:55 UTC When no renderer plugins are found, everything starts fine but nothing is of course displayed and the user has no clue as to what is happenning until he checks the list of loaded plugins and sees that there are actually no renderers present.

Having default renderer will help but with the plugin systems it can always happen than no plugin will be found for whatever reason.