headerbar: don't allocate both label and custom title

headerbar: don't allocate both label and custom title

It's only possible to have a label or a custom title, not both.
The size allocate code confusingly treats them as independent.

That is confusing, because, as the code is written, it makes it
look like the space for the custom title isn't getting accounted
for.

This commit else-ifies some parts of the size allocate code for
clarity.