[libadwaita] (9 commits) ...Add a GtkInspector extension

[Alexander Mikhaylenko <alexm src gnome org>]

Summary of changes:

  2a139f5... main: Add adw_is_initialized() (*)
  cb42371... style-manager: Load stylesheet without gtk-theme-name (*)
  bffad8a... settings: Rename has_color_schemes() to get_system_supports (*)
  7f08cd2... settings: Expose system-supports-color-scheme as a property (*)
  1af8ad7... style-manager: Anticipate system-supports-color-schemes cha (*)
  2bf9410... demo: Handle system-supports-color-schemes changes in runti (*)
  263dbac... settings: Add API to override system appearance (*)
  aa7e6f3... settings: Expose in the ABI (*)
  23f15dd... Add a GtkInspector extension (*)

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