Add a simple shim for status feedback.

[ntoll]

This PR introduces two things:

• A QStatusBar at the bottom of the application in which text can be placed to indicate an important event.
• A "shim" API in the logic layer called set_status which takes two arguments: a message string and (optional) duration (default = 5 seconds).

This is a temporary fix. I expect that status updates will still use the set_status method in future but we'll plug that into whatever the final status indication solution may be.

[redshiftzero]

looks good!