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.