revealer: Support minimum size of child

revealer: Support minimum size of child

Up until now when allocating the child it only used the natural size
while the measuring also used the minimum size, resulting in a clipped
child when animating if the child had different minimum size and
natural size. This was an obvious case when using labels that had
ellipsization.

This commit gives full allocation to the child by inverting the size
the revealer reduces from its animation progress.

Code done by Benjamin Otte.

Closes: https://gitlab.gnome.org/GNOME/gtk/issues/635