view: allow view to have more control over the toolbar menu

Currently we have this menu structure:

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1. New Folder/New Tab/Bookmark
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2. Zoom controls
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3. Undo/Redo
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4. Sort options
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5. Other view related controls
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The view creates 2-5, contained in a single GtkWidget - which is then
passed to the toolbar via the enclosing window slot. The problem is that
3 shouldn't be created or managed by the view as the controls in that
section of the menu are not related to the view. We'd like to move this
responsibility back to the toolbar, but that would mean the view must
pass multiple menu sections back to the toolbar (as 3 is in the middle
of the other view controls).

This change allows the view to pass multiple sections back to the
toolbar, using the new NautilusToolbarMenuSections structure. The files
view now passes 2 as a separate section to 3-5 (3 will be moved out of
the view in a future commit).

https://bugzilla.gnome.org/show_bug.cgi?id=764632