[gtk] (7 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:

  271354e... gtk: Add GtkSingleSelection
  b395589... stack: Provide a selection model
  eb0b1c5... stack switcher: Use the selection model
  a561a6c... stack sidebar: Use the selection model
  196b371... stack: Drop the position property
  e4b226c... stack switcher: Remove short-circuiting
  de92a16... stack sidebar: Remove short-circuiting
  7bc29ec... stack: Remove children in dispose
  3c60c92... stack switcher: Stop connecting to GtkStack::destroy
  1dfe5c8... sidebar: Stop connecting to the stack ::destroy
  a5d6156... stack: Manage children model ourselves
  fc17667... single selection: Fix a corner-case
  98040d8... stack switcher: Don't allow unselection
  9715542... a11y tests: Adapt to changes
  5ae47c5... testsuite: omit some selection properties

Commits added to the branch:

  b7e7bf1... gtk: Add GtkSingleSelection
  977c501... testsuite: omit some selection properties
  4099c77... stack: Provide a selection model
  e60cc9f... stack switcher: Use the selection model
  1cb9cb6... a11y tests: Adapt to changes
  0fc5d33... stack sidebar: Use the selection model
  3033e40... stack: Drop the position property