loginDialog: allocate children manually

The login screen is pretty custom full screen container and the standard
layout managers aren't really a good fit for the kind of layout that's
happening. This will be even more problematic with upcoming changes
to login banners, so we need to switch techniques.

This commit moves login dialog over to using a custom allocate handler
that has specific domain knowledge of the parts of the login screen
and where they go.

https://bugzilla.gnome.org/show_bug.cgi?id=703972
loginDialog: display banner message when disable-user-list=true

The login screen supports showing a banner message which admins
can use to mention login rules or disclaimers.

This message only shows up currently if the user list is enabled.
Most people who want to show a banner message also want to disable
the user list.

This commit moves the banner message to display when the user is
prompted for login credentials instead of when showing the user
list. It also adds a scrollbar if the message is too long.

https://bugzilla.gnome.org/show_bug.cgi?id=703972
loginDialog: use two column view if banner message long

Frequently banner messages are longer than can reasonable
fit in a one column view, which leads to a smooshed layout.

This commit changes the layout to a two column view, with the
banner on the left and the prompt on the right, if the banner
message is long enough that it can't fit well above the prompt.
If there isn't enough space for two columns then we keep the
one column layout but add scrollbars.

https://bugzilla.gnome.org/show_bug.cgi?id=703972