[gtk] (8 commits) ...Merge branch 'wip/baedert/for-master' into 'master'

[Matthias Clasen <matthiasc src gnome org>]

Summary of changes:

  2bc82da... inspector: Cache if we have ever constructed a window (*)
  136a3f6... stackswitcher: Expand child buttons (*)
  c2ab1f1... window: Fix mnemonics-visible getter+setter annotations (*)
  c24b787... gdktiff: Use guint32 instead of int32 (*)
  ae08aa3... constraintsolver: Remove a dead assignment (*)
  24415a6... cssimageurl: Explicitly check for local_error != NULL (*)
  1c2c356... widget factory: Scale down jpeg images (*)
  d66bc50... Merge branch 'wip/baedert/for-master' into 'master'

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