Refactor timed login and fix minor inconsistencies in gdm user-list

There were quite a few bugs in the timed login feature of gdm, it should always behave as expected now. If you notice any edge-cases I might not have covered please tell me.

The inconsistent top- and bottom-padding of the the user-list entries in gdm is fixed by hiding the timeout indicator by default, I also increased the padding to 6px according to Gnome Human Interface Guidelines.