loginDialog: add support for auth without username / fix Not Listed?

commit 93f072d1fcf578c4e7550bd755e84bd46abcd493 attempted to
add support for auth without a username to the login screen, but
do to a messed up rebase only partially added it.

This commit fixes that.

https://bugzilla.gnome.org/show_bug.cgi?id=706607
loginManager: Remove login manager versions of PowerOff/Reboot

These don't go through gnome-session, so they don't properly update
its state machine. We should use these in the future when we want to
use logind user sessions, but for now, they're just a trap.

https://bugzilla.gnome.org/show_bug.cgi?id=706612
endSessionDialog: Fix a warning

If _timerId is undefined/null, as it is by default, we will take this
path, and fail when trying to remove a source ID for undefined.

https://bugzilla.gnome.org/show_bug.cgi?id=706612
endSessionDialog: Don't error out if gnome-session hands us a dead inhibitor

Sometimes gnome-session hands us a bad object path for JIT inhibitors
it creates for XSMP clients. While this is a bug in gnome-session, we
shouldn't show an empty-looking dialog here.

https://bugzilla.gnome.org/show_bug.cgi?id=706612