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

This causes the endSessionDialog to send a ConfirmedRebootToBootOptions signal
to gnome-session instead of the normal ConfirmedReboot signal, actually
telling the boot-loader that it should show its menu the next boot is left
up to gnome-session.

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.
1 job for boot-options in 6 minutes and 42 seconds (queued for 1 second)
Status Job ID Name Coverage
  Source Check
passed #208520