WVV is not disposed when collapsed

[sbatten]

Testing #105764

From the sample, I simply added a onDispose listener which does not fire when the webview is collapsed even when retainContextWhenHidden is not set to true

[mjbvz]

This is the current design. The lifecycle of webview views is:

• expand/collapse maps to a visibility change.
• Switching visible views also maps to a visibility change.
• Show/hide via the context menu maps to dispose.

retainContextWhenHidden does not change when any of these events are fired, it instead controls when the underlying webview element is destroyed.

[mjbvz]

fbf81bd also expands the docs on the lifecycle