* Put hostext stuff in the HostExt sub-namespace * Tidy up system-specific stuff in PluginLoader * Make PluginLoader return a deletion-notifying wrapper which permits the library to be unloaded when no longer in use * Add PluginChannelAdapter * Make vamp-simple-host use PluginChannelAdapter, and use the PluginLoader for plugin-running task. Also some other enhancements to host