[libadwaita] (14 commits) ...example: Add a split button+button content page

[Alexander Mikhaylenko <alexm src gnome org>]

Summary of changes:

  907ae0f... stylesheet: Update menu button style (*)
  6eee9ae... Add AdwSplitButton (*)
  af2be5a... Add AdwButtonContent (*)
  0cd2703... stylesheet: Remove separator margin for non-menu popovers (*)
  f1b5704... stylesheet: Extract style for raised buttons (*)
  21336a5... stylesheet: Use toolbar style for header bars and action ba (*)
  f92e0ed... stylesheet: Reset toolbar styles for popovers (*)
  6502e96... stylesheet: Make GtkWindowControls buttons filled (*)
  2c0c15b... stylesheet: Make image buttons square (*)
  44dc0b3... stylesheet: Specificity bump for spinbutton buttons (*)
  be21dd5... view-switcher-button: Set spacing in css and match buttons (*)
  035456d... stylesheet: Add separator.spacer (*)
  2218040... doc: Mention the flat button changes in the migration guide (*)
  df50f94... example: Add a split button+button content page (*)

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