logout screen: show that the computer will enter the bootloader menu after reboot

Reason for the change

Right now the logout screen can tell you that you will enter the firmware setup screen after rebooting. This is a small MR to make it do this for rebooting to the bootloader menu too.

Test plan

I don't think there is a more proper way to do this right now (there's a systemctl option for this but I think it only applies if you request a reboot too), but I tested with busctl call org.freedesktop.login1 /org/freedesktop/login1 org.freedesktop.login1.Manager SetRebootToBootLoaderMenu t 0. (the value passed in is the requested timeout before the default boot loader entry will be chosen, 0 if there is no timeout or UINT64_MAX if you don't want to go the boot loader menu at all)
You can then run busctl call org.freedesktop.login1 /org/freedesktop/login1 org.freedesktop.login1.Manager SetRebootToBootLoaderMenu t 18446744073709551615 to undo this.

Screenshots or screen recordings

Before	After

P.S.: this is my first time writing Qt code, please be nice