Add command listener to external message listener

[bakkerthehacker]

This allows other extensions to trigger the same commands that are available as keyboard shortcuts.

Previously this was: https://github.com/photodiode/panorama-view/pull/79

The other extension panorama tab groups uses message external and (in older versions) allowed toggling the panorama by external message. See https://github.com/projectdelphai/panorama-tab-groups/pull/235