Baseline position in a row of widgets.
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 vertical 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 available space.
Align the baseline at the top
Center the baseline
Align the baseline at the bottom