Add a strictly centering header bar

GtkHeaderBar loosely centers its title widget and allow it to be offcentered if it allows the window to reach smaller sizes. It is great as it falls in line with our adaptive ideals, but it creates problems when we use adaptive containers as the title (HdyColumn or the in-progress best-fit container) as the container may take more width that its visible child would require and position its child according to itself and not to the headerbar's available space, making widget off-centered when space seem to be available for them to be centered.

A way to cirumvent that would be to fork GtkHeaderBar to give it the option to either strictly or loosely center its center child, with a smooth transition between the two states.

This would for example allow Web's URL bar to be centered in wide mode even with the close button visible, and to not be centered in narrow mode to not waste precious screen estate.

Edited Apr 16, 2019 by Heather Ellsworth

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Add a strictly centering header bar