Show tutorial at startup

[kaustubh-nair]

Currently this is a v1 implementation of @amanagr's #457 Follow-ups:

• [ ] Add option to skip tutorial at next startup
• [ ] Add options to modify config values.
• [ ] Modify config options on the fly
• [ ] Replace current loading text with LoadingView

[kaustubh-nair]

@mregdos, @preetmishra Thanks for the review. The tutorial text has not been finalized yet, but I think it's useful to have a discussion about this now. I've opened a topic here, you could weigh in. The point here is to have a basic mechanism to be able to show something like a tutorial-even if it later turns out to be something else entirely. That's why its called LoadingView rather than TutorialView

[neiljp]

@kaustubh-nair Thanks for following up on the previous work :+1: As we discussed on the stream, I think the first stage is to do the transition to having the Loading screen displayed in urwid itself, rather than externally. A minimal WIP version of this would probably only touch core.py and run.py (even initially building the UI element directly rather than adding a widget for it), which should help narrow down which elements of each commit of the full PR by @amanagr could go through first.

I've opened #628 as an issue to track this first stage of the process.

[zulipbot]

Heads up @kaustubh-nair, we just merged some commits that conflict with the changes your made in this pull request! You can review this repository's recent commits to see where the conflicts occur. Please rebase your feature branch against the upstream/master branch and resolve your pull request's merge conflicts accordingly.