Have better support for removing and adding control from Visual Tree

[hawkerm]

To curb memory usage, we try and remove all handles in the Unloaded event, as we don't know if that's because we're removed from VisualTree temporarily or for good.

In the case we get re-added to the VisualTree, then we need to reload all the handles and things that were removed. In v0.7, we made this work better for the editor itself so things wouldn't be unstable. But for things which require events and callbacks, there's some things which aren't getting rehooked back up properly still.

Not sure if this is a limitation of AddWebAllowedObject or something else. If we can eventually save state, then we could just deconstruct the webview, reconstruct everything on load again and restore state...

See Monaco Issue.