Save viewed state

[jacekkopecky]

This PR starts the work to save viewed state for diffs.

As discussed in #50 the state is saved in workspaceState so that we don't have to change the file.

I've split the work in three stages; the first is done

stage 1 – save basic boolean state

• [x] add toggleViewed message: filename -> viewedValue (bool for now)
• [x] instrument webview to send these messages
• [x] instrument extension to remember it in workspaceState
• [x] instrument extension to give the information to the webview when updating the view
• [x] instrument webview to reflect the remembered state on update

stage 2 – optional – make it work with multiple tabs open

• [ ] instrument extension to notify all relevant webviews about state changes (when the same diff is opened multiple times)

stage 3 – work nicely with changes in the diff file

• [ ] include hash of what was viewed in viewedValue
• [ ] where webview reflects the remembered state, if the current state doesn't match the hash, don't toggle as viewed but show a message "changed since last viewed"
• [ ] clear that message on toggle

"won't" requirements:

• [ ] make it a configurable option (because the content of the file isn't changing so I think it's a natural expectation that the state can be saved)

What do you think @caponetto ? I'm particularly not sure about where you would like some of the new interfaces and classes to live.

The first commit is just localized suggested renaming for clarity where we deal with elements inside a diff view, not the diff container itself.

[caponetto]

@jacekkopecky can you please rebase your branch with main?

[jacekkopecky]

Thank you @caponetto!

If you're happy with it for now, can you please look at the PR description and see if you like the rest of the plan? And would you like to merge first and have me continue in a next PR, or merge when the whole thing is done?

[caponetto]

@jacekkopecky good job here! I'll merge this PR already. Feel free to send others for stage 2 and stage 3, they are very welcome. :)