[gtk] (14 commits) Non-fast-forward update to branch fix-icontheme-invalidation

[Matthias Clasen <matthiasc src gnome org>]

The branch 'fix-icontheme-invalidation' 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:

  b9ffab5... Stop using the -gtk-icon-them style property
  1b677e3... css: Drop the -gtk-icon-theme property
  59b52db... css: Treat icon theme change as SOURCE change
  a204c62... widget: Add a system_setting_changed vfunc
  d43b95f... settings: Use system_settings_changed
  539b928... image: Invalidate icon helper when icon theme changes
  49dc084... Remove an unused include

Commits added to the branch:

  e818bf8... docs: migration guide fixes (*)
  654af88... Change the approach to menu section titles (*)
  1edfd80... Adwaita: Some styling for menu section titles (*)
  c09b5ea... Merge branch 'menu-section-title' into 'master' (*)
  ae4c6c6... Stop using the -gtk-icon-them style property
  61e538f... css: Drop the -gtk-icon-theme property
  f87a9c2... widget: Add a system_setting_changed vfunc
  29c816e... window: Use gtk_system_setting_change
  675986c... settings: Call gtk_system_setting_changed
  703e8e2... icontheme: Call gtk_system_setting_changed
  8bbd5a9... image: Invalidate icon helper when icon theme changes
  f83d5e7... spinner: Update when gtk-enable-animation changes
  66fb146... widget: Update when font settings change
  f142845... Drop gtk_style_context_reset_widgets

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