Add 'Automatic' Tab Style support
Add support for a new type of Tab Style called 'Automatic'. This tab style makes the GimpDockbook use the biggest actual tab style it can for its auto tab style dockables, based on its widget allocation. The tab style candidates for auto tab style are "Status + Blurb", "Status + Text" and "Status". A docked widget can also say that it wants to use "Icon" instead of "Status" for its auto tab style. The 'Tool Options' dockable does this. This is to be as backwards compatible with the old tab style setup, we make 'automatic' the default everywhere. We have quite a bit of dependency to internal layout code in GtkNotebook, but the current code should be pixel perfect and rather complete. Also add a basic regression test.
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