Borderless plugin UIs

This removes a border painted in the host's UI theme color around
any plugin UI.
This results in a much nicer look/feel for plugin-UIs that have their
own theme

gtk2_ardour/plugin_ui.cc

@@ -173,6 +173,7 @@ PluginUIWindow::PluginUIWindow (
 		if (h > 600) h = 600;
 	}
 
+	set_border_width (0);
 	set_default_size (w, h);
 	set_resizable (_pluginui->resizable());
 }