split-row: Use "both" for size group to avoid new GTK crash
New GTK changed layout algorithms for GtkBoxLayout, in particular adding an assertion that happens to trip for the "vertical" GtkSizeGroup we were using for CcSplitRow. "both" achieves exactly the same here in terms of looks, and doesn't crash, so use that.
See discussion on gtk!8068 (comment 2313048)
@felipeborges, doesn't really need review, but I'll leave it open if you want to look at what the heck was going on :P