gtkrevealer: support minimum size of child

GtkRevealer always allocates the natural size of the child so the
bin_window can take care of the animation with the full allocation of
the child.

However when GtkRevealer allocates the child doesn't take into
account the minimum size of the child. On the other hand it does take
into account the minimum size of the child when reporting the preferred
size to the parent of the GtkRevealer.
This behaviour clips the child of the GtkRevealer if the parent
allocates less than the natural size.

To fix this inconsistency, the patch makes the child adapt the
allocation of the GtkRevealer as long as it's not under the minimum
size of the child.

Closes #635
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00:09:26