Suggestion: ability for terminals to set their tab icon

#1685 Summary of the new feature/enhancement

The terminal currently provides a way for shells to specify the title:

In the world of web apps, both title and icon can be specified.

It would be useful for shells to be able to specify an icon for their tab for the same reasons:

• Sometimes visuals are more obvious than text. The node.js green gem, or the python logo, or some custom app icon all 'pop' visually more than the equivalent text does

• Terminals can update their icon to indicate specific conditions (a bell should be used for serious differences, but this could be used for more subtle ones, eg, showing a tunnel icon for an ssh tunnel running, then a shell icon if it stops and returns to the prompt)

• If we let terminals specify their title, why not let them specify their icon?