loginDialog: fix session menu visibility

The shouldShowSessionMenu function has a few bugs in it.
This fixes them.

https://bugzilla.gnome.org/show_bug.cgi?id=706153
Revert "background: fix asynchronous management of background loading operations"

This reverts commit 1020d8a0f8523a04d8336b1348388b8b242e414f.

https://bugzilla.gnome.org/show_bug.cgi?id=704646