[gtk] (11 commits) ...Merge branch 'matthiasc/for-master' into 'master'

[Matthias Clasen <matthiasc src gnome org>]

Summary of changes:

  6a3bd1c... window: default_icon_name can be null (*)
  b5077d0... Clarify the docs for gtk_widget_class_set_css_name (*)
  af944aa... widget: Add a missing nullability annotation (*)
  50310f7... TreeView: add missing nullability annotations (*)
  845d2e5... TreeModel: add missing nullability annotations (*)
  b47ea94... TextView: add missing nullability annotations (*)
  ec537d7... Text: add missing nullability annotations (*)
  9d52ca1... StringFilter: get_search fix nullable annotation (*)
  ad30ac3... Shortcut: add missing nullability annotations (*)
  475352f... StringList: add missing nullability annotations (*)
  4434889... Merge branch 'matthiasc/for-master' into 'master'

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