[gtk] (6 commits) Non-fast-forward update to branch stack-fixes

[Matthias Clasen <matthiasc src gnome org>]

The branch 'stack-fixes' was changed in a way that was not a fast-forward update.
NOTE: This may cause problems for people pulling from the branch. For more information,
please see:

 https://wiki.gnome.org/Git/Help/NonFastForward

Commits removed from the branch:

  e1919c0... stack: Provide a selection model
  0eb1a81... stack switcher: Use the selection model
  c20e58d... a11y tests: Adapt to changes
  0d9e3f5... stack sidebar: Use the selection model
  bf20049... stack: Drop the position property

Commits added to the branch:

  3cd52ce... stack: Add a GtkStackPage::visible property
  30987fc... stack: Provide a selection model
  544d473... stack switcher: Use the selection model
  b14a0a8... a11y tests: Adapt to changes
  37c741f... stack sidebar: Use the selection model
  52c5b4b... stack: Drop the position property