[geary] (8 commits) ...Merge branch 'mjog/folder-plugins' into 'mainline'

[Michael Gratton <mjog src gnome org>]

Summary of changes:

  254d107... plugins: Add support for folder plugins (*)
  c8dd0c9... Plugin.FolderContext: Add folder_selected signal (*)
  0e7d30f... Plugins.FolderContext: Add support for showing folder info  (*)
  a872c70... Plugin.Application: Add support for registering GLib Action (*)
  0c5c204... Plugin: Add new Button class to allow plugins to create but (*)
  27943e2... Plugin.InfoBar: Allow plugins to add a button to info bars (*)
  55a9bfa... Plugin.FolderStore: Allow plugins to get folders from varia (*)
  6869491... Merge branch 'mjog/folder-plugins' into 'mainline'

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