Size vfuncs always get non-null out variables passed, so no need to
check for NULL.

Instead, make the functions conform to the prototype, so that casting
isn't needed.

The refactoring in b9e37f8d broke this.

https://bugzilla.gnome.org/show_bug.cgi?id=724999

Do not constrain the child widget to its preferred size when the
revealer is larger.

https://bugzilla.gnome.org/show_bug.cgi?id=724742

Instead of Return value:

When animation is disabled, we use 'none' as the effective
transition type. So far, this transition type failed to change
the size request, causing the revealer to always take up the
space of the child, even when the child is not shown.

https://bugzilla.gnome.org/show_bug.cgi?id=719568

https://bugzilla.gnome.org/show_bug.cgi?id=712632

Instead of just setting them equal.

https://bugzilla.gnome.org/show_bug.cgi?id=706938

https://bugzilla.gnome.org/show_bug.cgi?id=702996

-Avoid defining variables in the middle of the block
-Include fallback-c89.c in gtkrevealer.c in place of math.h as
 fallback-c89.c includes math.h itself, and is needed for round()

Using a container for this is not necessarily the most
elegant solution, but it lets us reuse the animation
machinery in GtkRevealer.

This is a widget that can hide or show (ie reveal) its child
in an animated fashion.

This widget was initially developed in libgd.