[geary] (14 commits) ...Merge branch 'mjog/email-plugins' into 'mainline'

[Michael Gratton <mjog src gnome org>]

Summary of changes:

  9bb7b77... Plugin: Add EmailExtension plugin extension type (*)
  88c049e... Plugin.EmailStore: API doc fixes (*)
  e56b155... Plugin.EmailStore: Ensure email-sent is emitted by email pl (*)
  8d219b9... Application.MainWindow: Clean up builder and CSS class name (*)
  a7c06a3... Components.InfoBarStack: Use GObject style ctor (*)
  28e6b66... ConversationMessage: Convert to use InfoBarStack (*)
  6b483a8... Application.FolderContext: Move info bar impl to common loc (*)
  44806d2... ConversationListBox: Add convenience methods for email info (*)
  e29d937... Plugin.EmailContext: Add support for adding email info bars (*)
  8a36587... Plugin.EmailStore: Add email-displayed signal (*)
  e6b8785... Plugin.Email: Expose Geary.EmailFlags from the backing emai (*)
  d813934... Plugin.FolderStore: Add new list_containing_folders method (*)
  b2e1461... Plugin.SpecialFolder: Update to also handle drafts and outb (*)
  71b590f... Merge branch 'mjog/email-plugins' into 'mainline'

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