loginDialog: Use the same focus root for dialog and ctrl-alt-tab

Adding a group to the Ctrl-Alt-Tab popup will also add it to the
focus manager. Due to that, we currently end up with two focus
groups added for the login dialog - an explicit one for the entire
dialog, and an implicit one for the main content group.
When doing keynav, we ascend in the widget hierarchy from the
currently focused actor until we find a valid focus root, so
adding a children of the dialog as focus root breaks keynav to
any actors that are not inside the main content group.
The simple fix is to use the same group in both cases.

https://bugzilla.gnome.org/show_bug.cgi?id=684730