Unsubscribe from signals when clearing references

This fixes crashes with plugins calling setDirty() when the
UI is closed. `plugin` has already been reset before
the actual window is closed:

`PluginUIWindow::~PluginUIWindow()` runs, and destroys the window,
here `~VST3NSViewPluginUI ()`. This closes the VST3 view, which
triggers a `VST3PI::setDirty`.
At this point in time `~PlugUIBase()` has not yet run, so all
signals are still connected.

setDirty() ends up calling PlugUIBase::update_preset_modified()
which access `plugin` pointer, which is NULL.

gtk2_ardour/plugin_ui.cc

@@ -636,6 +636,7 @@ PlugUIBase::~PlugUIBase()
 void
 PlugUIBase::plugin_going_away ()
 {
+	drop_connections ();
 	/* drop references to the plugin/insert */
 	insert.reset ();
 	plugin.reset ();