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/UserAccountshttps://bugzilla.gnome.org/show_bug.cgi?id=767065
The UmAccountDialog class will be ported to gtk widget templates,
in doing so, we are decoupling here the gtkbuilder dependend ui
code from what will be the class code in the port.
Nothing changed in the join-dialog ui code itself.
https://bugzilla.gnome.org/show_bug.cgi?id=767065
