Command execution interface support

[ManuelSchneid3r]

Please see this. Comments are welcome.

[wez]

The linked issue is closed with a number of outstanding items todo.
I've been loosely following the discussions on the mentioned spec for a very long time.

If someone wants to produce a PR that shows the actual implementation impact, I'm open to discussing whether we should proceed with that PR, but I have no personal desire to wade into this or make changes to facilitate this at this time.

[ManuelSchneid3r]

The only requirement is this line. Gnome temrinal does however a lot of other stuff to check if it is set as default temrinal. Probably to let the user choose to make it the default one. However this is not strictly necessary.