GtkCellAreaContext used to compute the geometry of the cell view.
A group of cell views can be assigned the same context in order to ensure the sizes and cell alignments match across all the views with the same context.
GtkComboBox menus uses this to assign the same context to all cell views
in the menu items for a single menu (each submenu creates its own
context since the size of each submenu does not depend on parent
or sibling menus).