Add a side-by-side toggle

[jacekkopecky]

With this PR, DiffViewer windows have an icon in the navigation area to toggle line-by-line and side-by-side view.

The mirror icon is the best I found from among the standard codicon set.

It's the second icon in the top-right corner of these screenshots:

[jacekkopecky]

@caponetto the 500ms update delay forced in provider.ts feels too long when doing a quick refresh (e.g. when the side-by-side toggle changes).

I see that if it's completely removed, vscode doesn't show the "loading" message when opening a long diff file, which IIRC is the reason for the delay, but maybe we could reduce this to 100ms which seems to be enough for the "loading" message to show up?

[caponetto]

@caponetto the 500ms update delay forced in provider.ts feels too long when doing a quick refresh (e.g. when the side-by-side toggle changes).

I see that if it's completely removed, vscode doesn't show the "loading" message when opening a long diff file, which IIRC is the reason for the delay, but maybe we could reduce this to 100ms which seems to be enough for the "loading" message to show up?

@jacekkopecky 100ms looks good to me!

[jacekkopecky]

Thanks for the feedback, all done.