[gtk] (16 commits) ...Merge branch 'stack-fixes' into 'master'

[Matthias Clasen <matthiasc src gnome org>]

Summary of changes:

  317b9e6... gtk-demo: Fix the sidebar demo (*)
  16024fb... gtk: Add GtkSelectionModel (*)
  30386bb... gtk: Add GtkSingleSelection (*)
  dcdec4b... testsuite: Add selection model tests (*)
  ccb0a0f... testsuite: omit some selection properties (*)
  dc0750f... stack: Add a GtkStackPage::visible property (*)
  7b08d13... stack: Provide a selection model (*)
  8595665... stack switcher: Use the selection model (*)
  976664c... a11y tests: Adapt to changes (*)
  9255a12... stack sidebar: Use the selection model (*)
  916508f... stack: Drop the position property (*)
  cdb7294... selection tests: Plug a memory leak (*)
  2e1a204... Reorder tests (*)
  f496b4c... add more autoselect tests (*)
  78da8e4... Exclude some tests on too-old glib (*)
  46fba85... Merge branch 'stack-fixes' into 'master'

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