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

[Michael Gratton <mjog src gnome org>]

Summary of changes:

  a872fc3... Add libpeas to the build (*)
  c89766c... Add PluginManager class for client plugin support (*)
  526b120... Convert NewMessagesMonitor into a plugin context (*)
  e2e8e29... Add a libpeas Plugin.Notification extension, hook it up (*)
  486c2fc... Convert Notification.Desktop to a plugin (*)
  b54bbd4... Convert UnityLauncher to NotificationBadge plugin (*)
  2637b6c... Convert Libmessagingmenu to MessagingMenu plugin (*)
  5d13b8e... Ensure standard valac args are used when building plugins (*)
  d5e7c05... Move InAppNotification class to components package (*)
  c7c135c... Remove notifications preference now it is unused (*)
  73fd7e7... Plugin.Notification: Don't extend Geary.BaseObject (*)
  de6071e... Remove Play Sounds preference (*)
  5df1b97... Update online manual to reflect new preferences (*)
  41b72d5... Merge branch 'mjog/plugin-support' into 'mainline'

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