centerlayout: Fix measuring in presence of baselines

centerlayout: Fix measuring in presence of baselines

The measure logic (unlike the allocation logic) was enforcing strict
baseline alignment of child widgets even if no child widget had valign
set to baseline. This was causing GtkCenterLayout to request more size
than it actually needed.

Instead, bring the logic closer to that of GtkBoxLayout by introducing
explicit have_baseline and align_baseline variables. We track and report
baseline if have_baseline gets set, but it only affects our reported
minimum and natural sizes if align_baseline ends up set, which happens
if there's a child widget that has valign set to either one of the two
baseline values, and itself reports a valid baseline.

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