osc: Send /select/plugin plugin_id feedback

Whenever the active plugin changes (because the plugin changes or the
selected strip changes), the name, some other properties and parameter
values for the newly selected plugin is sent as feedback.  However,
there was no clear indication of which plugin was selected exactly.

Applications or surfaces that need to know the active plugin could track
the currently selected plugin by remembering the most recently selected
index, but unexpected plugin changes (e.g. when switching to a strip
with fewer plugins) and the asynchronous nature of OSC messages make
this more complicated than it should be.

By sending the active plugin id as feedback, clients do not have to
guess.

When no plugin is selected (e.g. on startup or when no strip is
selected) or the currently selected index is not a plugin (but a some
other kind of processor), a value of 0 is sent.

This uses the `/select/plugin` message, which is also used to change the
selected plugin. There is a slight discrepancy: the control messages
accepts a delta on the selected plugin, while the feedback sends an
absolute value.

libs/surfaces/osc/osc_select_observer.cc

@@ -486,6 +486,8 @@ OSCSelectObserver::plugin_init()
 		plugin_end ();
 		return;
 	}
+	_osc.float_message (X_("/select/plugin"), selected_piid, addr);
+
 	std::shared_ptr<ARDOUR::Plugin> pip = pi->plugin();
 	// we have a plugin we can ask if it is activated
 	proc->ActiveChanged.connect (plugin_connections, MISSING_INVALIDATOR, boost::bind (&OSCSelectObserver::plug_enable, this, X_("/select/plugin/activate"), proc), OSC::instance());
@@ -558,6 +560,7 @@ void
 OSCSelectObserver::plugin_end ()
 {
 	plugin_connections.drop_connections ();
+	_osc.float_message (X_("/select/plugin"), 0, addr);
 	_osc.float_message (X_("/select/plugin/activate"), 0, addr);
 	_osc.text_message (X_("/select/plugin/name"), " ", addr);
 	for (uint32_t i = 1; i <= plug_size; i++) {