[geary] (27 commits) ...Merge branch 'mjog/user-plugins' into 'mainline'

[Michael Gratton <mjog src gnome org>]

Summary of changes:

  c394611... Application.Controller: Make plugin manager public (*)
  655f1dc... Components.PreferencesWindow: Make application prop nullabl (*)
  1143bb0... Application.PluginManager: Convert ctor to RAII (*)
  f567777... Plugins: Use module name as plugin names (*)
  627bfb7... Application.PluginManager: Only load builtins if in install (*)
  0c526f6... Plugins: Support loading optional plugins (*)
  c290787... Add UI to preferences window for managing optional plugins (*)
  46e3ba7... Update releases notes to mention new plugin UI (*)
  a88a67b... Application.NotificationContext: Make API docs more concise (*)
  7ef6276... Plugin.Notification: Convert to an interface (*)
  25b527f... Application.PluginManager: Add notion of trusted plugins (*)
  45b519e... Geary.BaseObject: Use GObject ctor for ref'ing instances (*)
  899c1aa... Minor translatable string update for consistency (*)
  c50ab50... Application.ContactStore: Make ::close internal (*)
  18df946... FolderList.Tree: Remove use of a NotificationContext object (*)
  f72b135... Add stable public interfaces for plugins to use (*)
  03331fd... Application.PluginManager: Add close method (*)
  ee4bd11... Convert plugins to use isolated context model (*)
  ff7c0f2... Application.Client: fix expected GVariant types for actions (*)
  dbf9b75... Plugin.MessagingMenu: Rename plugin module name (*)
  835eb04... Plugin.MessagingMenu: Remove trusted status (*)
  154bb2d... Rework and clean up how plugin extensions work (*)
  df7889d... Make plugin activation and deactivation async and throw err (*)
  c72ce8d... Replace FolderList.Tree mail hooks with plugin (*)
  1963438... Plugin.DesktopNotification: Update notifications (*)
  4b53370... Plugin.NotificationBadge: Update plugin description (*)
  3f81b7d... Merge branch 'mjog/user-plugins' into 'mainline'

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