Allow the definition of public interfaces for plugins

As an example, the method `syncExtent()` from the globe plugin can be called from python.

    # Needs to be done before calling pluginInterface()
    from qgis import globe
    
    glb = iface.pluginInterface( 'libglobeplugin' ) # On windows it's just 'globeplugin'
    glb.syncExtent()

Question: should a "lib" prefix be stripped away for platform independence?