API: view management API for non-TextEditors

[siegebell]

Request: API support for managing non-TextEditor views. The functionality should support:

• show()
• hide()/close()
• move tabs/groups
• on-closed event (Notification when html preview window has been closed #12114, onDidCloseTextDocument for preview pane has significant delay #13623)
• on-focus-changed event (via onDidChangeActiveTextEditor or new onDidChangeActiveView)
• presence in visibleTextEditors or new visibleViews ([API] Provide Virtual Document in window.visibleEditors or similar #6941)

In particular, previewHtml should return an object that provides the above functionality,

Example use case: I create a preview for each open document of a certain file type. (The preview is derived from the current state of processing the document; not its contents.) Typically, the documents are all on the left column and their previews are on the right. When interacting with the document, its associated preview should become visible (without receiving focus); moving to a different column than the document itself if necessary. Without this kind of functionality, it easy for the user of my extension to become lost and confused when interacting with multiple documents.

[bpasero]

@jrieken fyi

[jrieken]

We have a dup for that but I am not finding it right now

[siegebell]

@jrieken I've seen a couple related issues, but none that request a comprehensive API for managing views.

I'll also amend this request to include views of any TextDocumentContentProvider. (Related: #10379)

[mjbvz]

I have a few markdown extension feature requests / bugs that all depend on this functionality, namely #8581

[StephenWeatherford]

We've got data associated with each opened HTML view, which could be very large. So we'd like to know when the tab is closed (not just deactivated) so we can dispose of it.

[jrieken]

closing as dupe of #15178