Made superclass destructors virtual, and introduced new `unimplemented' message

Destructors of `Plugin' and `CExporter' classes are now virtual to prevent undefined behavior when deleting subclass instances. Also, introduced new output message type `unimplemented' (besides `info' and `warn') to be used for functionalities that are not fully implemented yet (more visiblity to the user/developer).
Added capabilities for differentiating between normal and development plugins

Plugins can now set the `bDevelopmentPlugin' flag in their constructor. If this flag is set, and the `load-development-plugins' flag in the config file is set to `false', those plugins will not be loaded. This serves the purpose of ignoring plugins that are not necessary for (or might interfere with) normal operation. Either way, the user will be notified when a development plugin is loaded, or when it is ignored.
Added function for removing an interactive marker object from the server

By calling the appropriate function, a spawned instance of `InteractiveObject' can be removed from the interactive markers server again. Also, made this plugin a development plugin.