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

[Robert Pluim]

>>>>> On Thu, 17 Sep 2020 18:33:17 +0300, Eli Zaretskii <eliz@gnu.org> said:
    >> Now the question is: do we want to expose this to lisp

    Eli> I think so, yes.

    >> and if so, how?

    Eli> Some attribute of the binding, similar to :image and :vert-only, I
    Eli> guess?

    >> I mean, I have no idea if the macOS or MS-Windows tool bar have a
    >> similar feature.

    Eli> I don't know about macOS, but MS-Windows uses the native tool bar
    Eli> produced by our own code, i.e. it displays a Lisp string in a special
    Eli> window.

OK, so I took a look, and Iʼm not sure itʼs possible with the native
tool bar. We have '(space :align-to right)', but that just inserts
space up to a specified location, everything subsequent is
appended. In order to calculate the correct location, Iʼd need to know
the width of everything that came after the space, which only
redisplay can tell us, unless thereʼs a function Iʼve missed?
'string-width' doesnʼt take 'display properties into account.

macOS has something called a flexibleSpace toolbar item that might
serve, but Iʼm utterly ignorant of how the macOS code works.

Robert