[JetNews] List-detail support for main page

[alexvanyo]

Adds an initial list-detail support for the main page.

The overall approach relies on distinguishing between what the user sees as different screens, and what we implement as different navigation routes.

Navigation-wise, the "home" route is responsible for drawing the list detail screen, as well as the list screen and detail screen on their own (for small devices). Distinguishing between which one to show is now simply a matter of keeping track of whether the user is looking at the list, or a selected article.

Doing so allows meeting a couple of nice behaviors:

• We keep track of the user's state (scroll state on the list/detail, as well as which one they were looking at), even while changing which side we are looking at
• We avoid navigating as a side-effect of a size change
• We can make size changes reversible (mostly as a consequence of the previous point)

List:	Detail:	List and Detail:

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[manuelvicnt]

Left some initial comments, some of them are due to the lack of familiarity with the navigation flow you have in mind. We should try to work on simplifying it as the implementation detail looks tedious. Ideally, someone without the flow chart should be able to follow along easily.

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[alexvanyo]

@manuelvicnt I just pushed up another commit, which makes quite a few changes.

Most notably, I've removed the article route entirely (for now), so articles are now only displayed from the home route.

I also created a HomeRoute @Composable, which is the primary component responsible for determining which HomeScreen to display (based on the size of the screen, and the available state).

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[alexvanyo]

@manuelvicnt This is ready for another look! I've fully rebased on to the navrail PR, and have fixed up a couple things related to that rebase.

I also got to all of the comments you left, and added quite a bit of extra inline documentation around the approach.

[manuelvicnt]

Two nits but LGTM overall. Thank you!

[manuelvicnt]

Also @alexvanyo, can you add screenshots to the description of the PR for reviewers to get a sense of what's been implemented? At some point, we'll also have those mocks in the README. Thanks!

[JolandaVerhoef]

Is there a reason that you included the migration from List<Post> to PostsFeed in this PR? It's making it difficult to follow the changes related to that migration vs the actual changes made for the large screen support.

[alexvanyo]

@JolandaVerhoef Ah I apologize, that ended up being a refactor that happened partway through while cleaning everything up. Ideally that should have been pulled out into a separate PR that didn't touch the UI at all.

That ended up being changed to more nicely support having the topmost article selected by default. Previously, the post list was expecting a hardcoded list that was pulled apart into different sections, so the 4th post was arbitrarily chosen to be the "highlighted item." With the refactor to PostsFeed, those sections are now specified by the data object, so we can nicely choose the "highlighted item" to be selected by default.

[alexvanyo]

Thanks @manuelvicnt and @JolandaVerhoef !