linux: add support for check boxes

currently, getlantern/systray calls gtk_menu_item_new_with_label, which is why i think no check boxes appear when running on linux.

i think it needs to call gtk_check_menu_item_new_with_label when the menu item data structured has checked set to true.

the reference manual for this is here: https://developer.gnome.org/gtk3/stable/GtkCheckMenuItem.html#gtk-check-menu-item-new-with-label