Whenever a container has some form of natural row it may align
children in that row along a common typographical baseline. If
the amount of verical space in the row is taller than the total
requested height of the baseline-aligned children then it can use a
GtkBaselinePosition to select where to put the baseline inside the
extra availible space.
Available since: 3.10
Align the baseline at the top.
Center the baseline.
Align the baseline at the bottom.