[gtk] (14 commits) ...notebook: Use a box layout

[Timm Bäder <baedert src gnome org>]

Summary of changes:

  60f2e45... switch: Stop using a private struct (*)
  ed11f2a... bookmarksmanager: Stop exporting get_xdg_type (*)
  ce72154... Rename gtkbookmarkmanager.h to gtkbookmarksmanagerprivate.h (*)
  3a06394... headerbar: Don't update window buttons when unrealized (*)
  8ec1c86... widget: Inline build_finalize_assertions into only caller (*)
  32b84b8... native: Don't compare a pointer with 0 (*)
  a7b9a33... Add GtkCenterLayout (*)
  519967c... headerbar: Use a center layout (*)
  f0266cb... pixbuf: Add a missing return value transfer annotation (*)
  097f3c9... headerbar: Remove spacing property (*)
  adb77e1... appchooserdialog: Remove unnecessary gtk_widget_show() call (*)
  86cc7f6... fontbutton: Use a bin layout (*)
  6320bd5... fontbutton: Shorten clear_font_data a bit
  54ed31e... notebook: Use a box layout

(*) This commit already existed in another branch; no separate mail sent