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

This merge request 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 ConfirmedRebootMenu 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. The gnome-session side of this has its own merge-req: gnome-session!6 (closed)

Note I've tried implementing 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.