Generalize no-plugin-state from AudioUnits to all plugins

Replace AU_STATE_SUPPORT compile-time define with NO_PLUGIN_STATE and make it prevent plugin state setting, preset loading, preset saving and plugin state saving. Blocks on these actions exist partially in the backend and partially in the GUI (this latter class are not absolute, and should OSC or MIDI be able to drive Plugin::save_preset() the block could be circumvented). Set NO_PLUGIN_STATE if --freebie is used at waf-configure time