External Editor Auto Launch

When "use external editor" is selected a path to the external editor should optionally be specified and any sketches/documents opened in Arduino software should automatically be launch with the external editor. OR add a button in the upper-right corner over the shaded region to easily launch the file in the external editor (if possible, preferably as a tab in any existing window rather than a new window for when working with multiple files unless that is solely dependent on the external editor).

The issue names no files, tests, or entry points, so first locate the external-editor setting and the code that opens sketches or documents. Compare the two requested approaches and define done as either automatic launching with an optional editor path or a clear upper-right launch control, including behavior for multiple files.

Written by the indexing model from the issue text.