WIP: Fix GtkRevealer child allocation

The child widget allocation is never capped to the revealer actual allocation. Thus when the revealer gets small the scrolled window thinks it has much more space than there is and you cannot use the scrollbars.

Disclaimer: I think that's right but I don't know too much about widget size requisitions. It seems to work just fine in general and fixes issue #3408. Please, let me know if there are issues and I'll close the MR.

Thanks!