Make GtkCheckButton allocate all remaining space to it's child instead

Make GtkCheckButton allocate all remaining space to it's child instead
of limiting it to it's minimum size.

This fixes height-for-width labels inside a GtkCheckButton, for some
reason GtkCheckButton was limiting the child allocation to the child's
minimum request, probably for the sake of virtual left-alignment of
the child label to be beside the checkmark. This should be done by
other means if nescesarry.