* Webview API prototype 3
Part of #43713
Third try at refining the webview api. This pass reworks #44165. Major changes:
- Adds an `id` field to webviews. The id is provided by the extension and identifies the webview. It is used with the new event handling apis.
- Adds a new `onDidChangeActiveEditor` api. This is similar to `onDidChangeActiveTextEditor` but is also fired when you change webviews. It replaces the old `onFocus` and `onBlur` events on the webview itself
- Adds an `onDispose` event ot webviews. This is fired when a webview is closed by the user
- Perist webview state when the editor group changes. This is enabled for all webviews, not just those with keep alive.
* Throw error when trying to access disposed webview
* Improving webview documentation
* Clean up dispose management
* Throw if we receive a bad handle
* Move more event handling to input
* Simplify input updating
* Remove extra container property
* Fixing md security alert button
* Remove extra update container call
* Restore syncing of preview to active editor
* Fixing posting to webview
* Debounce preview updates
* Remove previewUri
* Enable direct window.postMessage instead of window.parent.postMessage
* Fixing scroll position not preserved when updating previews
* Revert parent.postMessage change.
Old behavior was correct
* Properly hide webview container on tab switch
* Make sure we only handle scroll events for the correct document
* Don't try setting negative scroll
* Revert vs code whitespace change
**Problem**
Editors use the `supportsSplitEditor` property to enable/disable splitting. Even if splitting is disabled, you can still split an editor by dragging it to create a new copy.
**Fix**
For non-splittable editors, make dragging always just move the editor to a new group
