bug#43405: Tool bar item doesn't align to the right edge

[Robert Pluim]

>>>>> On Mon, 21 Sep 2020 22:04:36 +0300, Eli Zaretskii <eliz@gnu.org> said:
    >> Does that sound like it would work? Is there a more direct way of
    >> calculating that width? (I got lost in all the various move_to
    >> functions).

    Eli> I'd first try to repeat what we do for :align-to support: insert a
    Eli> stretch glyph of a suitable width, computed using it->last_visible_x
    Eli> and the width of the image for the button that has to be
    Eli> right-justified.  See produce_stretch_glyph (except that most of it is
    Eli> not relevant, since :align-to supports a lot of functionalities).

OK, that would work, but then you could only right justify a single
item, which would be different from what you can do with the GTK tool
bar (and I see no reason to restrict this to tool bar buttons, I see
at least org-mode wants to right-justify headline tags)

Robert