1. Measure children for the actual allocated width in size_allocate;
2. When measuring horizontally for a specific size, subtract the height
   required for the bars before measuring the child.

This makes things work with inconstant-size content. Inconstant-size
bars are still not really supported; supporting them properly would
require doing something like what GtkBox does.

Signed-off-by: Sergey Bugaev <bugaevc@gmail.com>