Dialog to select proposal/directory when starting GUI

[takluyver]

I haven't tried to do any refactoring to take full advantage of this yet.

[JamesWrigley]

Things we discussed:

• Adding tests
• Open dialog from menu entry
• Creating a backend from the dialog if one doesn't exist

[takluyver]

Thanks @JamesWrigley for the review. I've made all the changes we discussed now; do you want to take another look overt the latest changes? The main change, other than the tests, is to the code for creating a DAMNIT DB and starting the backend if they don't already exist.

[JamesWrigley]

Hmmm, now I'm getting crashes when testing this on Maxwell and my local machine 🤔 Both with selecting by proposal and directory:

(amore) ➜  mid-2832 git:(open-db-dialog) amore-proto gui --no-kafka    
QThread: Destroyed while thread is still running
[1]    3384071 IOT instruction (core dumped)  amore-proto gui --no-kafka

[takluyver]

Gah, threads. I saw those errors while running the tests and fixed them there, but then didn't try again with the actual application. I'll take a look.

[takluyver]

OK, I think I've finally got the application, the local tests and the CI all working 😅 🤞 .

[JamesWrigley]

I didn't have quite enough time to test it myself, but I trust you and CI 😁 LGTM!

(P.S. I might also suggest squashing some of the commits before merging, but I leave that up to you)