Assets are not saved with LV2 plugins

Images or other assets referenced in a patch (potentially audio files, anything else?) should be saved along with the MFP JSON when an LV2 
plugin is created. Maybe this means a separate asset list editor? That would be handy when we move the file format to a single file that includes assets inside. 

For now, assets for plugins need to be in the -p search path to be found. 