[geary] (7 commits) ...Merge branch 'wip/fix-main-window-shortcuts' into 'mainline'

[Michael Gratton <mjog src gnome org>]

Summary of changes:

  e11e2bd... Convert prefs dialog to a HdyPreferencesWindow (*)
  fb14392... Update MainWindow shortcuts to be keybinding signal based (*)
  259ebe5... Re-implement single key shortcuts as a preference (*)
  82e7702... Update help with information about single key shortcuts (*)
  c4cba43... Update release notes with new features (*)
  dfe0b5c... Update online keyboard shortcuts help (*)
  01d34cc... Merge branch 'wip/fix-main-window-shortcuts' into 'mainline

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