Cleanup all loaded plugin instances when the PluginManager is deleted. In contrast to the previous implementation this should deal with multiple MarbleWidget instances correctly.
Move the duplicated plugin loading code into a common template method. Rename RenderPlugin::pluginInstance to RenderPlugin::newInstance for consistency.
Delete plugin loader if the loaded plugin is not a valid Marble plugin.
Increase RenderPlugin interface version. Note that you need to have cmake recreate the .moc files now to have the plugins loaded after a rebuild.

svn path=/trunk/KDE/kdeedu/marble/; revision=1181707