user-accounts: Introduce UmCarousel

UmCarousel is an horizontal container that contains UmCarouselItem
children. These items are paginated 3 at 3 at the time.

UmCarousel intents to act as controller for content containers. It
emitis the "item-activated" signal whenever an UmCarouselItem gets
activated (clicked).

It automatically activates the first UmCarouselItem of the current
vsible page.

The visibility of the go-back and go-next button is automatically
set based on the number of children.

These changes are according to the new User Accounts panel mockups
at https://wiki.gnome.org/Design/SystemSettings/UserAccounts

https://bugzilla.gnome.org/show_bug.cgi?id=767065