[gtk] (12 commits) ...Merge branch 'stackpage' into 'master'

[Matthias Clasen <matthiasc src gnome org>]

Summary of changes:

  ad9c2a6... stack: Convert child properties to a child meta object (*)
  ed3b966... builder: Allow specifying objects inline (*)
  addcf2e... tools: Split gtk-builder-tool (*)
  59152b8... builder tool: Rewrite the simplify command (*)
  1fb3feb... Document --3to4 (*)
  72b6644... builder tool: Operate on multiple files (*)
  4ace873... Update ui files to new stack syntax (*)
  e31f735... Some small additions to the migration guide (*)
  edd3f28... migration guide: Mention --3to4 (*)
  ce60089... assistant: Convert child properties to a child meta object (*)
  b675597... builder-tool: Convert assistant pages too (*)
  4763406... Merge branch 'stackpage' into 'master'

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