Add direct access to settings

[collinss]

It would be very handy to have an option in the right-click menu to open the settings window. You can do this by simply adding the following to MyApplet._init:

let settingsMenuItem = new Applet.MenuItem(_("Settings"), Gtk.STOCK_EDIT, Lang.bind(this, function() {
    Util.spawnCommandLine("cinnamon-settings applets " + UUID);
}));
this._applet_context_menu.addMenuItem(settingsMenuItem);

[mockturtl]

@collinss Shipped, thanks!