loginDialog: Open session/a11y menu on either button

loginDialog: Open session/a11y menu on either button

Users can choose whether the left or right mouse button acts
as the primary one, however that setting isn't carried over
to the login screen.

As the secondary button is unused in that context anyway, most
elements already accept either button. Do the same for the two
remaining elements.

Closes: https://gitlab.gnome.org/GNOME/gnome-shell/-/work_items/9243
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/4225>
(cherry picked from commit 8f80504eca99e4191fbc5561abe65429e80a7d26)