[gnome-builder] (3 commits) Non-fast-forward update to branch wip/chergert/session

[Christian Hergert <chergert src gnome org>]

The branch 'wip/chergert/session' 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:

  c7a7e16... WIP: editor: stub out editor session addin

Commits added to the branch:

  7aed518... workbench: delay addin cleanup until after saving session
  a0fe992... layout: add helper to add view with a given stack position
  59590cf... editor: implement session save/restore for editor