Add native external diff tool setting and action

[pbek]

Add a General settings option for the external diff tool path and a native note-list context menu action to diff selected notes directly.

[pbek]

26.5.12

• Added a native Diff selected notes note-list context menu action using a
  new External tools option in General settings for the external diff
  tool path, so the external diff tool can be configured and used directly in
  the application (for #3615)