loginDialog: Only highlight a single item at any time

loginDialog: Only highlight a single item at any time

The user list uses the same indication for hover and focus, so it
is possible for two items to be highlighted at the same time. Using
different styling would improve the situation, but only to some
extent - the user would still need to figure out which highlight
corresponds to which activation method. So instead, copy the
approach we use in popup menus and use a single property for
highlights that is updated by both focus- and hover changes.

https://bugzilla.gnome.org/show_bug.cgi?id=772284