implement 'show titlebar' window option

This was implemented on gtk in #587. This is at least somewhat complicated on mac; it isn't 100% clear what it should correspond to. If there is no window border on mac that means the window also is not resizable or moveable by default. Is that the intended behaviour? this needs to be clarified.

cc @Psykopear: exactly what is the intended behaviour of a window that has show_titlebar false? can that window be resized or moved? We may need to slightly rethink this..