endSessionDialog: Support rebooting into the bootloader menu aka ("Boot Options")

endSessionDialog: Support rebooting into the bootloader menu aka ("Boot Options")

This implements the "Alt" behavior for the "Reboot" button as outlined in
the design here: https://wiki.gnome.org/Design/OS/BootOptions

Note I've tried implemeting this with the AltSwitcher class from
js/ui/status/system.js first, but that puts the button in a St.Bin()
which causes the button to think it is the only button on the dialog
and makes it have rounded corners on both of its bottom corners.

https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/199