[gtk] (12 commits) ...Merge branch 'matthiasc/a11y-buildable' into 'master'

[Matthias Clasen <matthiasc src gnome org>]

Summary of changes:

  4ea2a66... a11y: Add parsing code for accessible values (*)
  9e4316b... a11y: Add an "accessibility" section to the builder XML tre (*)
  7702670... docs: Document the "accessibility" custom tag (*)
  8075632... a11y: Handle relations in UI files (*)
  313ba02... color editor: Add accessible labels (*)
  e050a26... builder: Expot _gtk_builder_boolean_from_string privately (*)
  fce455a... a11y: Parse booleans in ui files as expected (*)
  677ea5f... a11y: Reuse builder parsing for enums (*)
  f981ab7... Remove erroneous preconditions (*)
  0fddf73... a11y: Add some parser tests (*)
  c783e9a... Treat tristate as an enum when parsing (*)
  0c49f73... Merge branch 'matthiasc/a11y-buildable' into 'master'

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