Make the separator for the :guilabel: role a configurable character

Is your feature request related to a problem? Please describe.

With #6738, the separator character 'TRIANGULAR BULLET' is not escaped anymore for unicode capable latex engines. Unfortunately, this depends on the font having this character. I use latex_engine = 'xelatex' and the Adobe Source Sans font family which does not have this character.

Describe the solution you'd like

I would like a configuration option to override the default character.

Describe alternatives you've considered

I currently use this code snippet in my conf.py:

def setup(app):
    '''
    modify the `menuselection` role to replace the default character
    `TRIANGULAR BULLET` with `RIGHTWARDS ARROW` as the font that we use
    does not have this character.
    '''
    from docutils.parsers.rst import roles
    from sphinx.roles import GUILabel

    class CustomMenu(GUILabel):
        def run(self):
            self.text = self.text.replace('-->', '\N{RIGHTWARDS ARROW}')
            return super().run()

    roles.register_local_role('menuselection', CustomMenu())

which works as I want, but it feels like a hack.