Initial support for baselines
This modifies the size machinery in order to allow baseline support. We add a new widget vfunc get_preferred_height_and_baseline_for_width which queries the normal height_for_width (or non-for-width if width is -1) and additionally returns optional (-1 means "no baseline") baselines for the minimal and natural heights. We also add a new gtk_widget_size_allocate_with_baseline() which baseline-aware containers can use to allocate children with a specific baseline, either one inherited from the parent, or one introduced due to requested baseline alignment in the container itself. size_allocate_with_baseline() works just like a normal size allocation, except the baseline gets recorded so that the child can access it via gtk_widget_get_allocated_baseline() when it aligns itself. There are also adjust_baseline_request/allocation similar to the allocation adjustment, and we extend the size request cache to also store the baselines.
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